INFLUENCE OF EXTERNAL STIMULI. LIGHT 229 



as they do, I have already said something on p. 218. Leitgeb found 

 that relationships similar to those observed in the formation of arche- 

 gonia occur in the development of shoots on apogamoits prothalli of 

 ferns; such shoots of course arise asexually and independently of the 

 archegonium T . The side to give rise to these shoots is determined entirely 

 by the illumination ; and they always arise upon the shaded side like 

 the primordia of archegonia. If the illumination be directed upon the 

 other side after these shoots have once arisen, no further formation of shoots 

 can as a rule be brought about in the side that is now shaded which was 

 formerly the illuminated side, because the existing shoots have claimed 

 all the plastic material of the prothallus ; this is different from what 

 happens in the formation of archegonia. A reversal in the position 

 of formation of the shoots by a change in the direction of illumination 

 can only be effected if such change be made when the primordia of the 

 shoots on the first shaded side have not passed beyond the earliest stages. 

 A very peculiar case is that in which the members of one and the same 

 plant can be distributed over different sides of a prothallus on the 

 one side the shoot with first and second leaf, upon the other the first 

 root. This takes place in the prothallus of Pteris cretica if it be 

 illuminated from below at a stage when it has formed young primordia 

 of shoots but no root. Such a case is unquestionably very rare, and 

 Leitgeb's statements do not make quite clear whether the influence is 

 here one affecting merely the exit of the root upwards or its origin 

 upon the upper side. 



To what has been said above we may add two cases in which the 

 'polar' differentiation in the germ-plant proceeding from the spore is 

 determined by light. Most spores have polar differentiation ; about its 

 cause we know nothing. We see it in the difference between the 

 anterior end and the posterior end in swarm-spores, through which in 

 those Algae which possess a fixed thallus a polar differentiation obtains 

 in the young plantlet, because the swarm-spore fixes itself by the anterior 

 end. The spores of many Bryophyta and Pteridophyta all those in which 

 they are tetrahedral indicate plainly the position at which the germ-tube 

 will issue and this is the ' shoot-pole.' The factors which determine the 

 disposition of the poles in germination of radial spores are mostly unknown, 

 it is only in Equiseturn and some Fucaceae that we have any information. 



The spherical spores, full of chlorophyll, of Equisetum 2 exhibit a 

 similar construction in all their radii. In germination a small biconvex 



1 Leitgeb, Die Sprossbildung an apogamen Farnprothallien, in Ber. cl. deutsch. hot. Gesellsch. iii. 

 (1885), p. 169. 



2 Stahl, tlber den Einfluss der Beleuchtungsrichtung auf die Teilung der Equisetnm-sporen, in Ber. 

 d. deutsch. hot. Gesellsch. ii. (1885), p. 334; Buchtien, Entwicklungsgeschichte des Prothalliums 

 von Equisetum. Inatig. Diss. Rostock, 1887; Id. in Bibliotheca botanica, viii. Cassel, 1887. 



