EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 353 



(Fig. 372a), belong to the Devonian Age. Both of these suggest that 

 there is no need to call in " metamorphosis " to account for the early 

 establishment of spore-bearing parts. These fossils show that in them 

 tetrad-division has been an earlier and more stable feature of the Hof- 

 meisterian cycle than the evolution of leaves themselves. (See Chapter 



XXXIV., p. 545-) 



The sporangia of the Higher Plants vary in size, disposition and 

 number. But however inconstant in these points, there is essential 

 uniformity in the development of their contents, which when mature 

 are represented by the pollen-grains (microspores) and the embryo- 

 sac (or megaspore) (Chapters XV., XVI.). In either case a fundamental 

 feature in their production is the tetrad-division of the spore-mother- 

 cell. (See pp. 282-3, and 296-7.) This event involves Meiosis, and is 

 inherent in spore-formation, not only in the Higher Flowering Plant 

 but also in such of those lower in the scale as possess sexuality. In 

 fact syngamy and meiosis may be regarded as correlated phenomena, 

 which recur constantly, not only in the Higher Plants but also in those 

 lower in the scale — as will be seen in Chapters XXI. to XXXIII. 

 This is in itself further evidence of the homology of sporangia at large. 



In the lower forms of vegetation, more clearly than in the specialised 

 Flowering Plants, another category of parts is seen, viz. the gaynetan- 

 gia, or sexual organs, which produce respectively the male and female 

 sexual cells or gametes. In all but the most primitive plants the game- 

 tangia may be readily distinguished as male and female. The former 

 are called antheridia and they produce spermatozoids, which are usually 

 numerous from each ; the latter are called oogonia or archegonia, and 

 they produce one or more eggs respectively. The gametangia are 

 described for the leading types of Pteridophyta and Thallophyta in 

 Chapters XXI. to XXXIII. In the more primitive types, and par- 

 ticularly among the Algae, the gametangia are all alike : but in the 

 more advanced they differ according as they are male or female. Hence 

 it may be concluded that they are all homologous, though differen- 

 tiated in relation to sex, and to the act of syngamy. The events of 

 syngamy or fertilisation, and of meiosis or reduction appear as com- 

 plementary features, alternating in the Hofmeisterian Cycle (Fig. 427). 

 This is seen most obviously in the Archegoniatae (Chaptejs XXX. to 

 XXXII.) . In the Algae and Fungi it is less obvious, owing to their 

 smaller size, and their more rudimentary sexual differentiation. On the 

 other hand, in the Phanerogams the process of syngamy is obscured, 

 partly by simplification of the parts directly involved in their sub- 

 aerial fertilisation. Notwithstanding these differences, as seen in the 



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