The dispersal unit of Dacrycarpus dacrydioides (A. Rich.) de Laubenfels (Podocarpaceae) and the significance of the fleshy receptacle

FOUNTAIN, D. W., HOLDSWORTH, J. M. & OUTRED, H. A., 1989. The dispersal unit of Dacrycarpus dacrydioides (A. Rich.) de Laubenfels (Podocarpaceae) and the significance of the fleshy receptacle. Dacrycarpus dacrydioides (formerly Podocarpus dacrydioides ) is an arborescent gymnosperm endemic to New Zealand. The high water content (43%) and sensitivity of viability towards desiccation, suggest the seeds are of the 'recalcitrant' type. The 'fruits' comprising a seed borne on a fleshy receptacle arc shed in large numbers. The development of the seed precedes the full development of succulence in the receptacle and at maturity the seed has a high relative water content (RWC) relative to the receptacle. Within the maturing reproductive unit, the receptacle buffers the seed against the effects of water stress. After shedding, seeds are rapidly desiccated in moving air, and viability is impaired below approximately 80"' RWC and abolished at 34", seed RWC. The presence of the receptacle during drying confers resistance to desiccation-associated damage. Five phases of desiccation sensitivity are distinguished in recognition of the protective role of the receptacle. It is suggested that the advantages associated with prolonging seed viability may have contributed to the evolutionary development of succulence in the reproductive unit. This might be considered as a selection pressure in a manner similar to the proposal that such fleshy structures are associated with seed dispersal by birds.     

The Concept of Minimum Leaf Number

Several quotations of minimum leaf numbers for floral initiationare given in the literature, but it was thought desirable toobtain further evidence that they do represent, as claimed byPurvis and Gregory for rye, obligatory vegetative growth precedingthe attainment of Klebs's Blhreife. The effects of mineral starvationon leaf number, as in Purvis's original experiments, were thereforetested with Xanthium, Eupatorium adenophorum, and Celosia argentea.In each of these, near-constant leaf numbers are characteristicof plants induced to flower rapidly by continuous short-daytreatment. In each, treatments such as starvation which retardvegetative growth do somewhat reduce the numbers. In Xanthiumindeed, the concept of Blhreife is superfluous, for the 8 leavesto be accounted for could be explained as the primordia presentin the seed plus the extra leaves initiated during daylengthinduction. For the other two plants, on the other hand, thesum of these is still many short of the required number, andfor them a concept of Blhreife expressed by leaf number is retained,but with the admitted doubt whether all factors controllingtheir flowering are yet known.     

The Leaf Movements of Soybean, a Short-day Plant

If Bünning is correct in supposing that illumination ofa short-day plant during its skotophases prevents its generalmetabolism from reaching the normal ‘skotophile maximum’,leaf movements, too, should be diminished in non-inductive daylengths.‘Biloxi’ soybean is a well-known short-day plantand its leaflets show pronounced movement between day and night.However, measurement of the angle between the day and nightpositions of the leaves did not reveal any difference in longand short days.     

The Effect of Length of Dark Period on the Amplitude of Leaf Movement in Bauhinia monandra

In Bauhinia monandra, a short-day plant with a critical daylengthfor flowering of about 12 hours, there is a progressive decreasein the amount of nastic leaf movement as daylength increases.When the daylength is increased from 18 to 18.5 hours, however,there is such a sudden decrease as to suggest the existenceof a critical dark period at that daylength. The parallel withthe critical dark period in the photoperiodism of some short-dayplants lends some support to Bünning's endogenous rhythmexplanation of photoperiodism. All the same, in Bauhinia thesame rhythm cannot very well control both leaf movement andphotoperiodic response, for the observed critical daylengthsdo not agree.