INFLUENCE OF EXTERNAL STIMULI. LIGHT 



237 



question still requires experimental investigation. The dorsiventrality of 

 the capsule of the Buxbaumieae is the most striking. Diphyscium foliosum 

 (Fig. 117), as well as the species of Buxbaumia, possesses an oblique capsule, 

 one side of which is flat and the other convex, when seen in transverse 

 section, and the former is the illuminated side. In young radial sporogonia 

 of Diphyscium foliosum, which I cultivated in unilateral illumination, 

 I found that the flattening always takes place on the illuminated side, 

 which is here that part of the capsule-wall through whose movement 

 (brought about by rain-drops) the discharge of the spores through the 

 peristom-fringe is facilitated. In Fig. 117, which represents a longi- 

 tudinal section through a sporogonium not yet quite ripe, but still pos- 

 sessing calyptra and operculum, the difference between the illuminated 

 and the shaded side is quite evident ; the sporo- 

 gonium at a later stage is not so erect as is shown 

 in the drawing. The flattening upon the illuminated 

 side has also a relation to assimilation, seeing 

 that the sporogonia during their development are 

 able to assimilate by their chlorophyll-tissue, but 

 in Diphyscium the relation to the scattering of 

 the spores is the more important. 



We must include a number of marine Algae 

 amongst plants in which the position of the branches 

 is determined by the influence of light. In Halo- 

 pteris filicina, for example, the branching is commonly 

 distichously alternate ; radially branched examples 

 however also occur. Berthold : has investigated 



o 



experimentally the behaviour of Antithamnion cru- 

 ciatum, one of the Florideae. He found that plants 

 cultivated in the horizontal position had their ' leaves ' 

 laid down only in the horizontal plane, whereas on the positive heliotropic 

 shoots they arose in all directions. Callithamnion corymbosum behaves 

 in the same way. Fig. 43, of Antithamnion (Pterothamnion) Plumula, 

 shows that its branching takes place normally in one plane, but occasionally 

 there are found lateral twigs both on the upper and on the under side, 

 although these are usually in a reduced condition. In other bilateral 

 algae, the dorsiventrality is ' inherent,' that is to say, it is not caused by 

 external factors. 



In the regeneration of many Siphonieae- the directive influence of 

 light is very apparent. The ' leaves ' of Caulerpa are capable of producing 



FlG. 117. Diphyscium folio- 

 sum. Longitudinal section 

 of a stem bearing a sporo- 

 gonium. 



1 Berthold, Beitr. zur Morphologie und Physiologic der Meeresalgen, in Pringsh. Jahrb. xiii. 

 Many of the details in this paper are incomprehensible to me. 



2 Noll, liber den Einfluss der Lage auf die morphologische Ausbildung der Siphoneen, in Arb. 

 d. bot. Instituts in 'Wurzburg, iii (1888), p. 466. 



