748 



LECTURE XL 11. 



obtain a fixed position in order that it may maintain a definite direction with 

 respect to the external world. Finally, it should be remarked that in Fig. 432 

 also the gelatinous mass referred to {s I) is still to be seen, enveloping the whole 

 germinating apparatus. 



^^'hen at length the whole of the store of nutritive material in the macrospore is 

 exhausted, and the young plant has produced several roots and leaves, it becomes 



free from the prothallus and other parts con- 

 cerned in its origin, and developes into a 

 perennial plant. 



We have here a first step in the transition 

 from the germination of a spore to the develop- 

 ment of a seed. The two parts prothallium and 

 embryo represent, as we see, the two alternate 

 generations which we found completely sepa- 

 rated in Equisetum and the ordinary Ferns, each 

 as an independent living plant. Obviously, we 

 might also suppose the prothallium to be de- 

 veloped in the interior of the macrospore and not 

 protruding, and that it, together with the embryo 

 developed in it, remained enclosed in the thick 

 membrane of the macrospore ; in this case the 

 germinated macrospore would constitute the 

 essential part of the seed of a Phanerogam. 



The behaviour on germination of those 

 Lycopodiacege which possess two kinds of spores, 

 however, actually accords with the idea just put 

 forward. Fig. 433 shows at A the macrospore of 

 the genus Isoetcs and at B the prothallus with its 

 archegonium <?, developed entirely in its interior. 

 These points come out still more clearly 

 in the germination of the spores of the genus 

 SelagincUa. The sporangia here arise in the 

 axils of the leaves at the ends of the foliage 

 shoots. Fig. 434 A shows the fertile shoot of 

 a Sclaginella looked at from the outside ; B is 

 the same in longitudinal section. The sporangia 

 situated in the leaf-axils are obvious at once, 

 and it is observed that those on the right side 

 contain only four (only three are visible) large 

 spores, whereas the sporangia on the left side 

 If the macrospores and microspores are sown 

 separately on a suitable damp substratum, both develope, it is true, but no subse- 

 quent fertilisation occurs : the unfertilised macrospores give rise to prothallia with 

 archegonia, it is true, but no embryo arises in these. This fact, which can be 

 confirmed in the case of the most different Cryptogams, is in so far of great value as 

 it is not always feasible to observe directly the entrance of antherozoids into the 



Fig. 432. — Marsilia salvcitrix. Longi- 

 tudinal section of macrospore, prothallium, 

 and embryo (X about 60) : am starch-grains 

 in spore ; i inner membrane, ruptured above ; 

 ex epispore, with prismatic structure ; c 

 cavity beneath the protruded diaphragm on 

 which the basal layer of the prothallium is 

 situated ; pt prothallium ; ivh its root-hairs ; 

 a archegonium ; y foot of embryo ; w its root ; 

 s its stem-apex ; * its first leaf, which distends 

 the prothallus ; si gelatinous envelope of the 

 spore ; at first it forms the funnel above the 

 papilla, and it still envelopes the prothallium 

 (50 hours after sowing). 



contam numerous microspores. 



