478 HETEROMORPHISM AND ALTERNATION OF GEXEHATIONS. 



which, having one sort of spore only, are termed lioriiosporous) is of interest, since 

 it leads on to the condition prevailing- in Flowering Plants. In these the alternation 

 of generations is not obvious, no recognizable and detached sexual generations being 

 seen. But on certain shoots of flowering plants {i.e. in the flowers) sporangium- 

 bearing leaves are borne; these are the stamens and carpels respectively. The 

 sporangium borne by the stamen is the pollen-sac, and the contained pollen-gi-ains 

 are the microspores. The microspore or pollen-grain, when it germinates on the 

 stigma (or in the micro pyle, in Conifers, cf. p. 418) forms a pollen-tube, which 

 contains the male fertilizing element, corresponding to a spermatozoid. Of course 

 the conditions of fertilization in the Flowering Plant are altogether different from 

 those obtaining in the Vascular Crytogams, and motile swimming spermatozoids 

 are no longer produced. The sporangium borne by the carpel, on the other hand, is 

 the ovule, and the embryo-sac contained within the ovule is regarded as the macro- 

 spore. As a rule but one macrospore is met with, but in certain Amentacese (e.g. 

 Cavfnnus, see fig. 314a, p. 412) more embryo-sacs (macrospores) than one are present. 

 In the Flowering Plant the macrospore is not shed from its sporangium (ovule), but 

 germinates in situ, forming an egg-apparatus {cf. fig. 316 and p. 417), and certain 

 other cells, which ultimately form the endosperm. These structures are regarded 

 respectively as corresponding to the archegonium and female prothallium of such a 

 heterosporous Vascular Ci-yptogam as Selaginella. If the content« of the embryo-sac 

 in Gymnosperms (see p. 415) and in Angiosperms (see p. 417), respectively, are com- 

 pared with the female prothallium of Selaginella or other heterosporous Vascular 

 Cryptogam, it will be seen that the Gymnosperm shows the greater agreement. 

 In it the archegonia are still quite recognizable as such, though these now take 

 part in quite a diffei'ent type of fertilization. In all Flowering Plants (Gymnosperms 

 and Angiosperms) as opposed to the Vascular Cryptogams, the microspores produce 

 pollen-tubes in the vicinity of the ovules, and these penetrate to the embryo-sac 

 (macrospore) and fertilize the egg-cell. Consequently the counterpart of the arche- 

 gonium is not exposed, as it is in Vascular Cryptogams, in which a free-swimming 

 spermatozoid has to gain entrance. 



Thus we see that in Flowering Plants the female prothallium or sexual genera- 

 tion is hidden away in the embryo-sac, and is never an independent structure. 

 This fact is correlated with the different manner of fertilization which obtains in 

 Flowei'ing Plants as compared with Vascular Crj'ptogams. 



In the Mosses the sexual organs are formed at tlie tips of little leafy shoots; 

 lertilization is much as in Ferns, and from the fertilized egg a new (asexual) 

 generation arises. This generation, known in Mosses as the sporogonium, consists 

 of a stalk (the seta) terminating in a spore-capsule above. The sporogonium 

 develops within the archegonium on the sexual generation of the Moss. The base 

 of the seta penetrates some distance into the fertile Moss-shoot, and is in this 

 way able to absoi'b nourishment. As the sporogonium elongates, the archegonial 

 wall stretches with it up to a certain point, then it breaks across transversely and 

 the upper portion is i-aised up on the capsule as a sort of hood or extinguisher (the 



