Phototropism and Pigment Production in Sordaria in Relation to Quality of Light

Experiments with Sordaria funicola are described giving evidencethat in phototropism of the perithecial necks and in the light-inducedproduction of yellow-orange intracellular pigment, light ofwave-lengths below 550 mµ is principally effective. Thisagrees with earlier findings on the sensitivity of light-stimulatedspore discharge in this fungus.     

Hooker Lecture 1974: Convergent evolution in aquatic fungi: the tetraradiate spore

Fungi are mostly terrestrial. Less than 2% of the species are aquatic. These include primitive species with zoospores (Chytridiomycetes and Oomycetes) and re-migrant types mostly in Ascomycetes and Fungi Imperfecti (including Hyphomycetes) with a very few in Basidio-mycetes. Aquatic Hyphomycetes are abundant on decaying leaves (especially of broad-leaved trees) in well-aerated streams and rivers. Their conidia are hyaline and mostly of unusual form. In many the conidium is tetraradiate consisting of four long arms diverging from a common point. In the different genera the tetraradiate conidium develops in very different ways. In some it is a phialoconidium; in others a thalloconidium. In some attachment to the conidiophore is by the tip of one of the four arms; in others near their point of divergence. In some all arms develop simultaneously; in others in succession. The wide range of developmental geometry suggests convergent evolution. The tetraradiate aquatic spore also occurs in Basidiomycetes. The only two higher Basidiomycetes (both marine) with submerged sporophores have tetraradiate basidiospores. The world-wide distribution of aquatic Hyphomycetes is discussed. Their general ecology is also considered. Concentrations of 1000 to 10,000 conidia per litre may be reached in small rivers in autumn. Attention is paid to the possible biological value of the tetraradiate aquatic spore. The evidence suggests that this is connected with the problem of initial anchorage in the turbulent conditions of a stream.     

Carbon Dioxide and Fruiting in Sphaerobolus

In cultures bathed in air deprived of its CO² fruiting in Sphaerobolusis greatly reduced or fails completely. However, increasingthe concentration of CO² in the air to 1or 5 per cent tendsto reduce fruiting as compared with normal air. The stimulatoryeffect of CO² in normal air appears to be exerted at the stageof development when sporophore primordia are being produced.The CO² does not seem to require the immediate presence of lightto exert its effect. It has not been found possible to replacethe effect of CO² by substituting Krebs-cycle organic acids.Using ¹⁴CO² it is shown that ¹⁴C is incorporated in the myceliumboth in light and in darkness.     

Sporophore Production in Sphaerobolus with Special Reference to Periodicity

Further observations on temperature and fruiting are reported.The minimum and optimum values depend on the time when observationsare made. The effect on discharge from sporulating culturesof illumination interrupted by dark periods (i-ii days) is considered.If the dark period does not exceed a day, harge recommenceson the second and subsequent days of renewed illumina if itis 2–4 days, discharge occurs on the second day afterreturn to light but not thereafter; if the period of darknessexceeds 5 days, discharge does not mmence on subsequent illuminationuntil a completely new crop of sporophores is formed. Experiments are reported in which it is shown that at 20°C there is an interval of about I day between the onset of adark period and the inhibition of discharge related to it. Thisperiod is increased to a day and a half at 10° C. At thistemper an alternation of roughly equal light and dark periodseach day leads to dis in the dark periods and not in the lightperiods as at 20° C. Experiments are reported which support the view that the long-termof discharge in cultures under constant conditions relates toinhibition of development of primordia by maturing sporophores.     

An Analysis of Spore Discharge in Sordaria

Sordaria fimicola can develop mature perithecia from which sporesare discharged when grown in darkness or in light. Under conditionsof alternating dark and light (12 hrs.: 12 hrs.) each day, sporedischarge is periodic with a low rate during the dark period,succeeded by a gradual rise to a relatively high rate in thelight period followed by a decline before the onset of the nextdark period. There is no trace of an endogenous rhythm. Transferfrom darkness to light always leads to an increase in the rateof discharge, and from light to dark to a decrease. The heightof the peak of discharge rate attained in light following adark period seems to be related to the length of the precedingdark period. Experiments with light of different colours but of roughly thesame energy value show that it is the blue rays that are mainlyeffective. From cultures of filter-paper yeast-extract mediuman orange pigment can be extracted with a maximum absorption,in the visible spectrum, at 470 mµ. It is possible thatthis is important in connexion with the sensitivity of the fungusto blue light.     

Light and Spore Discharge in Ascobolus

Spore discharge in Ascobolus crenulatus occurs both in the lightand in the dark. In a 12 h light: 12 h dark daily regime discharge-ratehas peaks in the dark periods, due apparently to light stimulationwith about half a day's interval between stimulus and response. Using a ‘spore clock’ the course of discharge hasbeen followed for a single apothecium on changing from darknessto light. Exposure to light (500 lux) of wave-lengths around400, 440 and 460 mµ immediately causes ’puffing‘,whilst light of longer wave-length (504 and 580 mµ) hasno effect. Change from darkness to white light has no immediateeffect, but there is a delayed stimulation with a marked increasein discharge-rate 10–14 h later. Simultaneous illuminationof an apothecium, which has been in darkness, by blue light(420 mµ, or 440 mµ, 500lux) and yellow light (580mµ, 500 lux) does not result in puffing. The yellow appearsto prevent the blue light from exerting its effects.     

Sporophore Development in Sphaerobolus: Effect of Blue and Red Light

Experiments are described which indicate that, although lightof short wavelength (400–500 mµ) is necessary forover-all sporophore development, during the last days of theprocess light of longer wavelength (640–720 mµ)accelerates development more than blue light (400–500mµ) does. It is shown that at 20° C this effect ofthe longer rays of light begins to be exerted 8 to 10 days aftersporophore initiation and only a very few days before maturity.