478 HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



which, having one sort of spore only, are termed homosporous) 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-grains 

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

 stigma (or in the micropyle.. 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 most Flowering Plants are altogether different from 

 those obtaining in the Vascular Crj^ptogams, and motile swimming spermatozoids are 

 very rarely 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. 

 Carpinus, 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 Cryptogam as Selaginella. If the contents 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 diflerent 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. In the case of Ginkgo and the Cycads, 

 however, these tubes liberate motile sperniatozoids before the archegonium is 

 reached, so that a vestige of the Cryptogamic method is still retained. 



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 

 Flowering Plants as compared with Vascular Cryptogams. 



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

 fertilization 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 absorb 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 raised up on the capsule as a sort of hood or extinguisher (the 



