;i6 CONCLUSION 



// thus appears that comparison of the several phyla, as represented 

 both by their fossil and their modern representatives, leads in each case 

 towards the recognition of a primitive type, and that its construction in 

 the several phyla has certain features in common. The chief of these 

 are the definition of axial polarity in the first initiation of the embryo : 

 the continued apical growth : the radial construction of the shoot : the 

 origin of the appendages laterally from the axis by enation, and in strictly 

 acropetal order : a protostelic structure of the conducting system of the 

 axis, and a leaf-trace composed of a single strand, which comes off from 

 the protostele with the minimum of disturbance of its structure. The 

 appendages were from the first of two kinds which were closely associated 

 together : bracts or leaves, and spore-producing members : the structure 

 of these, and their relations to one another and to the axis, varied in 

 the different phyla, and gave them their distinctive characters : but a 

 whorled arrangement of the bracts was prevalent in early small-leaved 

 forms, while they commonly held a subtending relation to the spore- 

 producing members. A body such as that sketched appears to have been 

 common for all the early Pteridophytes, and constituted the primitive shoot. 

 There is no clear indication, beyond comparison based on the facts of 

 embryology and of mature structure, how such a body was in the first 

 instance produced ; but this leads to the hypothesis put forward in 

 Chapter XL The sporophyte, thus constituted, probably arose originally 

 as a structure of limited size, and unbranched, upon a prothallus of 

 considerable dimensions, and producing Homosporous Spores. From it, by 

 branching of the axis, by differentiation of vegetative and propagative 

 regions, by amplification of the leaves and spore-producing members, by 

 adoption of an alternate leaf-arrangement as the leaves enlarged, and 

 by expansion of the vascular system to meet these additional require- 

 ments, all the known homosporous types may be understood to have 

 originated. But as explained in Chapter XLVL, the adoption of 

 Heterospory, and of the Seed-Habit supervened later. This, while it has 

 led to the final independence of the Land-Flora as regards external fluid 

 water for the completion of its Life-Cycle, has brought as a secondary 

 consequence a wide-spread reduction. 



The final gt>al of all organic development is the establishment of 

 new individuals. The evolutionary story of the sporophyte illustrates this 

 in two distinct ways. In the prior and non-specialised homosporous forms 

 large numbers of germs are produced: those are individually small, and 

 ill provided with nourishment, but they make up for deficiency of method 

 by their large numbers. The larger their number the better the chance 

 of survival and spread of the race : consequently amplification of the whole 

 sporophyte is the leading characteristic of these earlier and simpler types ; 

 it was carried out either by multiplication of appendages individually small, 

 as in the microphyllous types, or by enlargement of individual appendages, 

 as in the megaphyllous types. It was in these homosporous forms that 



