EMBRYOLOGY 



343 





which the prothalli grow, and they are attached by hairs which project 

 in all directions. Here again a fungus plays an important part in the 

 nutrition, which is exclusively saprophytic. The prothalli reproduce readily 

 by gemmae, as also by progressive decay, which separates the ultimate 

 branches as distinct individuals. The sexual organs are borne upon the 

 upper surface of enlarged branches of the thallus, and are always accom- 

 panied by paraphyses. 



Such different types of prothallus, when studied separately, appear 

 widely divergent : and at first the underlying unity of their construction 

 was less appreciated than the differences which they show ; so little indeed 

 that Bruchmann, to whose labours so many of the important facts are 

 due, was disposed to make those differences the basis of a division of 

 the genus Lycopodium into distinct groups, or even genera. 1 But Lang,, who 

 had simultaneously with him been 

 at work on the prothallus of Z. 

 clavatum? pointed out clearly the 

 relation of the divergent types 

 to one general plan, recognising 

 especially how the prothallus of 

 L. Selago, one of the species 

 described by Bruchmann, gives 

 the clue to their connection. For 



itS prothallus appears tO be Variable 



j c j , /T -,. 



in ItS mode Of development (Fig. 



1 80). It is usually a pale under- 

 ground body; but at other times 



it grows above ground, and is coloured a full green. The spores appear to 

 germinate either at the surface or below it. The form of the prothallus is 

 determined largely by the soil in which it develops : thus, the elongated 

 cylindrical form is usually found in firm ground, though less deeply 

 buried than in the annotinum-\.y$Q : the thallus seems, in fact, to stretch 

 upward as though to bring as near to the surface as possible the seedling 

 unsuited for subterranean growth. The subterranean prothalli may be 

 simple, or be branched so as to take a coral-like form. In more open 

 soil, however, and especially near to the surface, the prothalli are more 

 compressed and flattened. Each prothallus tapers off as in the other 

 types at its lower end into a conical point, which indicates where it 

 issued from the spore, while towards its upper end the sexual organs are 

 formed. In the half-saprophytic prothalli, grown to the surface of the soil, 

 the conical form similar to that of other types is clearly seen (Fig. 181) : the 

 saprophytic lower region, the meristem, and the crown bearing the sexual 

 organs and paraphyses holding the usual positions. 



1 Ueber die Prothallien und die Keinipflanzen mehrerer europaischen Lycopodien, 

 Gotha, 1898, p. 108. 



-Annals of Botany ', xiii., p. 279. 



FIG. 180. 



Prothalli of Lycopodium Selago, bearing seedlings. 

 o, o shows the level of the soil, and the seedlings in their 

 development show varying proportions so that the first. 



a 



