706 CONCLUSION 



megasporophylls are the simplest of all, the microsporophylls are still 

 of considerable size, while those of the modern Cycads are much smaller, 

 though their megasporophylls show less extreme reduction. Such com- 

 parisons appear to indicate with unmistakable clearness that in the course 

 of descent a reduction of sporophylls has followed upon the establishment 

 of the Seed-Habit, but that it came gradually, and is not to be held as 

 a simple or direct example of correlation. 



The essential point in the Seed-Habit is the retention of the megaspore 

 within the tissues of the parent plant till after fertilisation : on this has 

 followed, in the first place, the ultimate achievement of a higher degree 

 of independence as regards fertilisation ; and secondly, the opportunity of 

 continued nutrition of the embryo by the parent plant up to an advanced 

 age. Both of these are important steps in the establishment of a Land- 

 Flora, and must be briefly considered. Taking first the question of method 

 of fertilisation, it may be remarked that the differentiation of sex as 

 evidenced in heterospory is in itself no point of adaptation to a land- 

 habit : while it brings an advantage in the superior nutrition of the female 

 spore, it imposes a fresh difficulty in fertilisation, viz., the necessity during 

 germination of a near juxtaposition of the microspores and megaspores, 

 bodies which have a distinct source of origin : the more distinct the origin 

 in space, and in time of production, the larger will be the number of 

 microspores requisite to ensure a reasonable probability of fertilisation. As 

 a matter of observation the number of microspores in the Pteridophytes is 

 habitually maintained according to the plan of the original homosporous 

 sporangium, and it may be in Seed-Plants also, so long as their distribution 

 is by no specialised method, and so long as juxtaposition with megaspores 

 is only a matter of chance. This is exemplified in the Pteridosperms and 

 in Cycadeoidea, and in less complete degree in the Cycads. But in the 

 higher forms of Seed-Plants the specialised methods of transfer of micro- 

 spores, and especially those by animal agency, have led to economy, so 

 that a reasonable certainty of fertilisation is secured with a smaller output 

 of microspores. This theme may be so fully illustrated by well-known 

 examples from the Flowering Plants that it requires no further explanation 

 here. But as against the difficulty of securing juxtaposition of the mega- 

 spores and microspores during germination may be set the adoption of 

 siphonogamy, which followed ultimately upon the Seed-Habit. A precision 

 previously unknown was thus introduced into the act of fertilisation, so 

 that once juxtaposition of spores was secured, fertilisation followed with 

 a high degree of certainty. This reduced and finally abolished the motile 

 stage, and so removed the critical process of fertilisation from its primitive 

 dependence on the presence of external fluid water. The adoption of 

 siphonogamy was the last adaptive step of prime importance in the 

 establishment of a Land Flora upon a permanent basis of suitability to 

 external circumstance : and the high degree of certainty of the resulting 

 fertilisation still further favoured economy of pollen-production. 



