ALTERNATION OF GENERATION 553 



and later, specialised methods of spore-distribution. These rose to 

 high efficiency in the higher Mosses (Figs. 355, 367). A well-formed 

 epidermis with stomata is found in some of them, while beneath 

 this lie assimilating tissues as well ventilated as in the leaves of 

 Vascular Plants (Fig. 364). Nevertheless, the simple form, the 

 limited apical growth, the absence of appendages, and above all the 

 want of any direct connection with the soil, stamp even the most 

 elaborate sporogonium as an only partially efficient structure. To 

 achieve higher development it would be necessary to break away 

 from so restricted a plan, which in itself is only practically possible 

 where the gametophyte shows high elaboration, so as to supply the 

 nourishment the sporophyte cannot wholly acquire for itself. 



The Homosporous Pteridophytes are free from such restrictions. 

 The features in w T hich they show superiority to the Bryophyta as 

 spore-producing plants are : (i) an unlimited capacity for apical growth ; 

 (ii) the possession of lateral appendages ; (iii) a direct access to the 

 soil by a root-system ; (iv) an improved conducting system ; (v) a 

 well-ventilated photosynthetic system, with elaborate external form, 

 and more complete differentiation of vegetative from propagative 

 regions of the plant ; (vi) the formation of numerous distinct spor- 

 angia ; and (vii) the production of the sporangia not simultaneously 

 but in succession, or even delayed, so as to spread the physiological 

 drain over a long period. Possessed of these features the sporopfr 

 develops as an independent, self-nourishing organism, unlimited in 

 plan, in period of life, and in power of spore-production. It is the 

 sphere of Special Morphology to trace the lines along which these 

 various features may have been acquired. But the result of them is 

 seen in varying proportion and efficiency in any ordinary Fern, or 

 Lycopod, or Horse-tail. These are characteristic examples of the 

 primitive Vascular Plants of the Land. They depend upon the vege 

 tative development of a freely-rooted sporophyte for their legitimate 

 success, while still retaining their homosporous state. In point of 

 size the acme of achievement of the Homosporous Pteridophytes 

 now living is to be found in the Filicales ; though they still show : 

 the most part a leafy shoot which serves general purposes, and is not 

 strongly differentiated into vegetative and propagative regions. 



It was the adoption of the heterosporous state and the retention of 

 the megaspore and its prothallus within the megasporangium, or 

 ovule, that paved the way for the full possession of the Land by Seed- 

 bearing Plants. Plants thus finally broke away from dependen 

 on external water for their fertilisation. Seed-production is carried 



