Experimental and Analytical Studies of Pteridophytes: XL. Factors in the Formation and Distribution of Sori in Leptosporanglate Ferns

In further studies of soral morphogenesis in common Britishferns, consideration is given to environmental and other factorswhich may be causally involved. Accounts are given of the normalcourse of sporophyll development in Dryopteris austriaca andof the effects of more or less extensive defoliation on theinception of sori in several ferns. Some anomalous developmentsin sporophylls of Blechnum spicant are described and illustrated.Various general ideas regarding factors involved in the onsetof the reproductive phase are discussed. It is concluded that,on the evidence which is admittedly very incomplete, the onsetof the reproductive phase in the fern sporophyte is determinedby the interaction of a number of factors, rather than by thespecial action of some particular factor. Because the nature,functional sorus is a complex and highly organized structure,its further investigation is considered to be a task of veryconsiderable magnitude.     

Experimental and Analytical Studies of Pteridophytes: XXVIII. Leaf Symmetry and Orientation in Ferns

An account is given of surgical treatments which may determinethe symmetry and orientation of leaf primordia in ferns. Inone series small deep tangential incisions, severing the incipientvascular tissue, were made immediately above very young leafprimordia or primordium sites: the primordia developed as leavesin all cases. In the other series two deep and wide tangentialincisions were made above a primordium (or site) so as to leavea ‘bridge’ of intact tissue between it and the apicalcell of the shoot. In some of the more actively growing apicesbuds were formed, even though the shoot apical cell was quiteundamaged; relatively inactive apices typically yielded leaves.It is concluded that the orientation and symmetry of a leafprimordium cannot be referred only to the direct action on itof a growth-regulating substance moving basipetally from theapical cell, but rather that these characteristic developmentsare mediated through the organization and physiological activityof the apex as a whole, the intact apical cell being a centraland essential element of the system.     

Experimental and Analytical Studies of Pteridophytes: XIX. Investigations on Marsilea

It is shown that in plants of Marsilea growing in aseptic culturereversion to juvenile leaves may be obtained by depriving thecultures of either sugar or mineral nutrients. Reduction toa protstele and other anatomical observations are also described. Juvenile leaves were produced by excised spices of mature plants,and by lateral branches developing on feeble decapitated plants.The problem of heteroblastic development is discused in relationto these and other observations.     

Experimental and Analytical Studies of Pteridophytes: XXI. Investigations on Marsilea

An account is given of the effects of various concentrationsof glucose, sucrose, and fructose on the development and morphologyof aseptic cultures of Marsilea sporelings. The response ofthe sporelings to a range of different sugars and to growthin liquid or on solid media is also described. The results are discussed with special reference to the relationshipbetween nutrition and heteroblastic leaf development. Otheraspects of the influence of various sugars on the growth ofplants in aseptic culture are considered in relation to theexisting literature.     

Experimental and Analytical Studies of Pteridophytes: XXXII. Further Investigations on the Effect of Undercutting Fern Leaf Primordia

An account is given of experiments in which young primordiaand primordium sites at the apex of Dryopteris aristata wereisolated basally and laterally by undercutting and deep radialincisions. The treatments had the effect of destroying or severingthe incipient vascular tissue underlying the primordium on itsabaxial and lateral sides. Provided the shoot apex had not beendamaged during treatment, neither buds nor centric (or radial)leaves were induced as a result of the experimental procedureemployed. On the contrary leaves with distended bases, showingan abnormally rapid rate of growth, were usually produced. Theresults obtained are against the view that a leaf-trace, differentiatingacropetally in advance, determines the formation of a primordium;but they support the thesis that the apical cell group exercisesa regulative effect on the growth and morphogenetic activitiesof the apical meristem and young primordia. The evidence alsosupports the view that the incipient vascular tissue is importantin morphogenesis in that it affords a pathway for the translocationof nutrients and hormonal substances.     

Experimental and Analytical Studies of Pteridophytes XXIV. Investigations on Marsilea: 4. ANATOMICAL EFFECTS OF CHANGES IN SUGAR CONCENTRATION

Sporelings of Marsilea, grown aseptically in various concentrationsof sugars, particularly glucose, have been investigated anatomically. It is shown that, corresponding to the features of the externalmorphology, the anatomy of the sporelings from the lower sugarconcentrations (1 and 2 per cent. glucose) has many characteristicsof the water forms of amphibious plants, while that of the sporelingsfrom the higher sugar concentrations (4 and 5 per cent, glucose)has the characteristics of typical land forms. A number of the differences between the two types of sporelingmay be referred to a change in the duration and direction ofcell-division in the rhizome-apex. Views as to the factors influencingthe origin of land or water forms are briefly discussed.     

Experimental and Analytical Studies of Pteridophytes: XXX. Further Investigations of the Formation of Buds and Leaves in Dryopteris aristata Druce

The influence of the shoot apex upon leaf and bud formationin the fern Dryopteris aristata has been investigated by furtherexperiments on puncturing the apical cell. When the apical cellgroup is damaged, leaf primordia, which may be orientated abnormally,continue to be formed on the meristem, but one or more budsmay also arise. The observations reported here indicate thata zone at the periphery of the apical meristem is particularlyreactive when the apical cell group is damaged, the majorityof buds being induced in this region. The extent of damage tothe apex may affect the sequence of organogenesis: when damageis extensive buds tend to be formed immediately, subsequentprimordia developing as leaves; when the damage is confinedto the apical cell, or extends to only a few of its segments,bud formation tends to be delayed. It is concluded that the effect of the apical cell on organformation is exercised through the growth and organization ofthe apex as a whole.     

Structure and Development of Stomata in Some Leptosporangiate Ferns

The structure and development of stomata are described for 17species of leptosporangiate ferns. In these species the maturestomata are anomocytic, diacytic, Pyrrosia(applied), tetracytic,suspended, or floating type. Stomata are present on both surfacesof floating as well as sub merged leaves of Azolla and Marsilea.In a few species twin stomata, arrested development, and persistentstomatal initials are present Floating stomata result from disintegrationof the anticlinal suspending wall in Pleopeltis, and by detachmentand displacement in Azolla pinnata. The development of stomatais haplocheilic, syndetocheilic, or syndetohaplocheilic. InCyathea spinulosa the guard cell nuclei divide amitoticallyand the resulting two daughter nuclei occupy the opposite polesof the guard cells     

Experimental and Analytical Studies of Pteridophytes: XXIII. The Induction of Buds in Ophioglossum vulgatum L.

A study has been made of the development of buds on isolatedroot and stem segments of Ophioglossum vulgatum. In both casesbuds may develop endo-genously. The anatomical details of developmentare fully described. The bearing of the facts on current theoriesof shoot initiation is discussed.     

Experimental and Analytical Studies of Pteridophytes: XVIII. The Nutritional Status of the Apex and Morphogenesis

The relation between the nutrition of the shoot apex and itsmorphogenetic activity is discussed with special reference tothe ferns. From an early stage in the individual developmentthe distal region of the shoot is dependent on supplies of nutrients(the term is used in a comprehensive sense) passed up from thebasal absorptive region. The obconical enlargement of the shootis attended by an increase in the size of the apical and sub-apicalregions; but whereas the primary morphogenetic activities ofthe apical region are closely comparable in small and largeapices, very pronounced differences in leaf shape and size andin stelar morphology may be induced according to the nutritionalstatus of the sub-apical regions. Large apices, which normallyyield large and complex leaves and an elaborate vascular system,can, by appropriate treatments, be caused to yield 'juvenile'leaves and a simple vascular system. The nutritional statusof the sub-apical regions is thus seen to be of considerableinterest and importance in studies of morphogenesis. A conceptionof the mode of distribution of nutrients in the shoot apex isdescribed and discussed. The need for physiological investigationsof the apex is emphasized and the scope for new work using themethods of tissue culture is indicated by reference to recentpublished work.     

Experimental and Analytical Studies of Pteridophytes: XXVI. Ophioglossum vulgatum: Comparative Morphogenesis in Embryos and Induced Buds

The sequence and rate of organ formation in embryos and inducedbuds of Ophioglossum vulgatum are compared. In the embryo, theappearance of the primary root considerably precedes the organizationof the shoot apex and the formation of the first leaf; but ininduced root and shoot buds the reverse is the case. With thelimited nutrition provided by the saprophytic underground prothallus,the first leaf of the embryo (actually the second leaf formed)does not appear above ground until after 7–8 years; butin buds induced in decapitated shoots, four to six new leavesmay be formed in 5–6 months. Completely defoliated shootsmay show a comparable rate of leaf formation. The conclusionis reached that while the fundamental pattern of developmentis the same in embryos and buds, the actual sequence in whichthe primary organs are formed, and their rate of formation anddevelopment, are referable to nutritional factors.     

Experimental and Analytical Studies of Pteridophytes XXII. Morphogenesis in Selaginella Willdenovii Baker: I. Preliminary Morphological Analysis

Changes of configuration at the shoot apex of Selaginella Willdenoviiduring the formation of branches and leaves are described anddiscussed in terms of growth.     

Experimental and Analytical Studies of Pteridophytes: XXXVII. A Note on the Inception of Microphylls and Macrophylls

In an attempt to shed new light on the nature of microphyllsand macrophylls, a study has been made of leaf inception inselected materials, including Psilotum, Tmesipteris and otherpteridophytes, and flowering plants such as Cuscuta with greatly‘reduced’ leaves. It is shown that the incipientprimordia of the small scale-like leaves of Psilotum and ofthe quite substantial, though microphyllous, laminate leavesof Tmesipteris are closely comparable and that there are noessential differences in the histological organization of incipientmicrophylls and incipient macrophylls. In parasitic speciessuch as Cuscuta, with small scale-like leaves, the organizationof the apical meristem and the inception of primordia are asin normal autotrophic species. The so-called ‘reduced’leaves and the microphyllous condition of some pteridophytesare attributable to a physiological-genetical limitation ofgrowth of primordia in the subapical regions of the shoot. Theconclusions which may be drawn from these observations are discussed.     

Experimental and Analytical Studies of Pteridophytes: XXXIII. The Experimental Induction of Buds from Leaf Primordia in Dryopteris aristata Druce

When a young leaf primordium of Dryopteris aristata is isolatedon a plug of tissue at any time prior to the formation of alenticular apical cell, the tissue may be induced to developas a solenostelic bud instead of a leaf. The result dependson the rate of leaf development. In some apices, leaf determinationmay take place during the first plastochrone; in others it maybe postponed until the end of the third plastochrone. Isolationof very young primordia by deep incisions in close proximityto the primordium may result in the cessation of meristematicgrowth and the formation of parenchyma. The vascular structureof the isolated primordia and the phyllotaxis of the inducedbuds are described, and the significance of the findings isdiscussed.     

Morphogenetic Studies on the Distribution and Activities of Leaf Meristems in Ferns

In very young sporophytes of Dryopteris and Osmunda, the leafprimordia originate very close to the shoot apical cell andshow early differentiation of an apical cell, rapid growth,and an early transition from distal to marginal growth. In successively older primordia of adult Dryopteris, a gradualelaboration in the size of the leaf apical cell takes placeand the greatest size is attained before lateral pinnae beginto be formed. With the formation of lamina, the apical cellgradually decreases in size and is transformed into the marginaltype of meri-stematic cell, when the leaf unrolls. In ferns with a homogeneous marginal meristem, which consistsof a uniform layer of cells with an equal capacity for growth,a simple, entire leaf is formed, e.g. Phyllitis and Platyceriumand where an initially homogeneous marginal meristem becomesheterogeneous, with a consequential differentiation of areasof unequal growth, a lobed or pinnate configuration, as in Blechnumand Lomnaria, or a compound leaf, as in Dryopteris, results. There are some indications of the inception of vascular elementsbeing due to the activity of functioning meristems, the processbeing a basipetal one.     

Experimental and Analytical Studies of Pteridophytes: XXXI. The Effect of Shallow Incisions on Organogenesis in Dryopteris aristata Druce

The hypothesis, that the regulative effect of the shoot apexand adjacent leaf primordia on incipient growth centres is exercisedby way of the incipient or prevascular tissue, has been testedby isolating leaf primordia and primordium sites, and also theshoot apex, by various shallow incisions. These incisions weresuch as to pass through the single superficial layer of meristemcells of the shoot apex, the incipient vascular tissue beingleft intact. Young primordia thus isolated by tangential incisionsdeveloped normally as leaves whereas with deep incisions budsare usually induced. Primordia isolated by four shallow incisionswhen the apex was not in active growth usually became dispersedand disappeared; the inception of prospective primordia couldalso be prevented in this way. The hypothesis relating to thefunctional activity of the pre-vascular tissue in morphogenesis,which is supported by the results obtained, is discussed inthe light of other recent experimental evidence.     

Experimental and Analytical Studies of Pteridophytes: XXXVIII. Some Observations on Spiral and Bijugate Phyllotaxis in Dryopteris aristata Druce

In an attempt to assess some of the factors which determineleaf arrangement at the shoot apex, a study has been made ofphyllotaxis in the fern Dryopteris aristata. It is shown thatclockwise and counter-clockwise spirals occur with equal frequencyboth in field collections and in adventitious and experimentallyinduced buds which arise on the rhizomes and apices of plantsgrown in the laboratory. In addition to the two common spiralarrangements, a form with bijugate phyllotaxis has been noted.It is concluded that the three types of leaf arrangement arenot under direct genetic control, but rather are dependent oncircumstances of growth at the apex.     

Experimental and Analytical Studies of Pteridophytes: XXVII. Investigations on Marsilea. 5. Cultural Conditions and Morphogenesis, with Special Reference to the Origin of Land and Water Forms

Experiments are described in which sporelings of Marsilea weregrown aseptically on media of similar nutrient value, but inwhich the osmotic concentration was changed by addition of mannitol,by alteration of the concentration of inorganic constituents,or by substitution of sucrose for glucose. Comparison is thenmade between the morphology of supported and submerged sporelings.Further experiments are described in which sporelings were transferredfrom high to low and from low to high sugar concentrations,from supported to submerged growth, and from glucose media tomedia containing mannitol. The principal hypotheses which seek to account for the characteristicfeatures of water plants are discussed in relation to the resultsof the above experiments. It is concluded that it is the waterbalance of the developing tissues, as determined by such factorsas the osmotic pressure of liquid media, the relative humidityof the atmosphere, or more generally, the diffusion pressuredeficit of the water of the environment, which leads to theappearance of the morphological features characteristic of landor water forms; nutritional conditions play only a subsidiaryrole.     

Experimental Studies of Apospory in Ferns

Apospory in Pteridium aquilinum was found to have several novelfeatures, in particular the rapidity of its occurrence. Outgrowthsfrom detached laminae were detected in as little as 3 days.There was no evidence that failure of co-ordination betweencells, senescence of the sporophytic tissue or unfavourableculture conditions effected the initiation of outgrowths asearlier workers have suggested. The response of Thelypterispalustris was notably slower and more in line with earlier reportsof apospory. Marsilea vestita appeared to be incapable of anyform of aposporous behaviour. The only feature common to all occurrences of apospory in leavesof wild-type ferns is the severance of the lamina from the mainbody of the plant. It is proposed that this may deprive thecells of regulatory substances which maintain the activationof sporophytic genes. The results are also considered in relationto the phase changes in homosporous and heterosporous plantsand the behaviour of fern protoplasts in culture. apospory, ferns, Pteridium aquilinum, Marsilea vestita, Thelypteris palustris     

Experimental and Analytical Studies of Pteridophytes: XXI. Sterile cultures of an abnormal growth from the gametophyte of bracken

The isolation and growth in sterile culture through sixteenpassages, each of 4 to 6 weeks' duration, of an abnormal growthisolated from a gametophyte of Pteridium aquilinum var. latiusculumis reported, and the nutritional requirements described. The cultures consist of green, photosynthetic, predominantlyfilamentous, but coherent, masses. Prothalli were regeneratedon five occasions during the first two passages only. Outgrowthsof whitish or pale green parenchymatoua tissue occur. Thesepossess a central strand of elongated cells, some of which becomedifferentiated into a core of tracheides. Prothalli of the strain in culture are haploid. When examinedsoon after the initial explantation, the filamentous parts ofthe cultures were diploid, but there has been a subsequent increasein the number of chromosomes, and each cell is now irregularlyaneuploid, with a chromosome number between triploid and tetraploid.