240 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



Preissia appears. So long as this plantlet is in the simple juvenile 

 stage, growing by means of a vegetative point with a ' two-sided ' apical 

 cell, it can be forced to revert to the formation of a germ-tube by bringing 

 it into light of low intensity (Fig. 118, 5). This germ-tube is able to 

 produce a germ-disk in light of higher intensity, and again reversion can be 

 induced. This process can be repeated at will. But whenever the young 

 plant has reached the ' mature form ' reversion to formation of germ-tubes 

 is no longer possible. This reversion is really not different from what 

 we have subsequently to consider in the chapter upon change of function 



caused by light 1 , nor from what has been already 

 mentioned that formation of roots is favoured 

 by want of light. 



I have satisfied myself through the exa- 

 mination of Plagiochasma Aitoniana that the 

 formation of its germ-tube cannot be hindered 

 by light of very strong intensity, but the 

 behaviour in germination of other liverworts 

 is somewhat different and requires more 

 accurate investigation. In Blasia pusilla, An- 

 thoceros, Alicularia, and some other leafy 

 Jungermannieae, either a germ-tube or a cell- 

 body may arise, according to Gronland ~ and 

 Leitgeb 3 . The germ-tube is produced according 

 to Gronland only if the spores are sown very 

 thickly, the cell-body is formed in Blasia 

 when the spores lie scattered, and we may 

 account for this by saying that external con- 

 ditions, and after the analogy of other cases the 

 intensity of the light, determine which kind 

 of germination shall take place. As in Preissia 

 too, the cell-body which develops in the ger- 

 mination of the spores of Blasia and Anthoceros 

 may again form a tube which behaves like a 

 germ-tube if it be placed in light of low in- 

 tensity (see Fig. 119, /-///). That the formation of a germ-tube here is 

 of as much advantage as the strong elongation of the axis of the seedling 

 from a seed that is sown too deep, is quite evident the germ-tube 

 endeavours to reach the light. We can speak of the formation of germ- 

 tubes then as an adaptation which in some liverworts has become inherited, 



1 See p. 255. 



2 Gronland, Memoires sur la germination de quelques Hepatiques, in Ann. d. Sc. Nat., ser. 4, i. 



'" Leitgeb, Untersuchungen tiber die Lebermoose, i. p. 52, and ii. p. 67. Leitgeb conjectures that 

 moisture is a determining factor upon the form of the pro-embryo ; but light is certainly the pre- 

 dominating one. 



FIG 119. Anlhoceros. /, cell-mass 

 produced in germination. 77, cell- 

 filament developing from the cell- 

 mass. 777, further stage of the cell- 

 filament. After Leitgeb. 



