INFLUENCE OF EXTERNAL STIMULI. LIGHT 



239 



f 



BRYOPHYTA. 



In the Bryophyta we have to deal with like phenomena. On the 

 one hand the appearance of the individual moss-plant on its protonema 

 is associated with the existence of a more intense light than is required 

 for the luxuriant growth of the protonema itself 1 ; and on the other hand 

 the form of the protonema itself is influenced by light. Both conditions 

 will be here spoken of together. 



HEPATICAE. In many liverworts the germinating spore forms in the 

 first instance a germ-tube which in the Marchantieae flattens out at its apex 

 to a pluricellular germ-disk from which the young plantlet sprouts. In 

 other forms, the germ-tube passes at its apex directly into the plant 2 . 

 Germination takes place so far as it has been investigated only in the 

 light. The germ-tubes are positively heliotropic in diffuse light, and 

 Leitgeb has pointed out 

 that their length de- 

 pends upon the intensity 

 of the light ; in feeble 

 light they are longer 

 than they are in stronger 

 light, and further, in 

 feeble light there is laid 

 down neither the primor- 

 dium of a germ-disk nor 

 of a leafy plant. I may 

 mention here as an ex- 

 ample the germination 

 of Preissia commutata 

 (see Fig. 1 1 8) 3 . The 

 germination of the spore takes place in bright light in the manner shown in 

 Fig. ii 8, 1. A short germ-tube is produced, and out of its uppermost cell 

 there proceeds a flat germ-disk at right angles to the direction of the light. 

 This disk in many cases consists of two cells, in others of four. One 4 of 

 these cells will become the apical cell of the young plantlet, which is at 

 first very simple in structure, being composed of only one layer of cells, 

 and it is only later that the characteristic structure of the thallus of 



3. 



FlG 118. Preissia commutata. Half diagrammatic representation of the 

 germination of spores, partly from Hansel's figures. In 1 and 5 the spore 

 is shown below. 7, a germ-disc is produced at the end of a short germ- 

 tube. 2 and 3, two germ-disks seen from above ; I I, the first segment wall ; 

 2 2, the second segment-wall ; from one of the four quadrant cells the young 

 plant usually proceeds, but as is shown in .*) the plant may develop from 

 one of the cells resulting from division by the first segment-wall ,v apical 

 cell of young plant; this cell has developed again a germ tube after 

 forming five segments in 5. 4, germ-disk in optical cross-section with the 

 primordiuin of a young plant on the left. 



1 There can be no doubt that the heliotropic movements also of the pro-embryo are ' attuned ' to a 

 less intense light. 



2 See Part II of this book. 



3 See Schostokawitsch, in Flora, Ixxix (Erganz.-Bd. 1894^, p. 358. 



4 Leitgeb says the primordium of the plantlet arises in the most illu ninated quadrant of the germ- 

 disk in the Marchanlieae ; but as the disk is spread out at right angles to the light-rays all the 

 quadrants should be equally illuminated. This is well seen in Preissia where the disk consists 

 sometimes of onlv two cells. 



