ALTERNATION OF GENERATIONS 489 



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

 nourishment the sporophyte cannot wholly acquire for itself. 



In various ways the Homosporous Pteridophytes show the result 

 of such emancipation, and they may perhaps have originated from 

 some source like the sporogonium seen in certain Bryophyta. The 

 features in which they show superiority to these as spore-producing 

 plants are (i) an unlimited capacity for apical growth ; (ii) the pos- 

 session of lateral appendages ; (iii) a direct access to the soil by a root- 

 system ; (iv) an improved conducting system ; (v) a greatly elaborated 

 Photo-synthetic system, partly dependent on external form, partly 

 on 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 sporophyte 

 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 have probably been acquired. But the result of them 

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

 Lycopod, or Horsetail. 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 for 

 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 reten- 

 tion of the female spore and its prothallus within the megaspor- 

 angium or ovule, that paved the way for the full possession of the Land 

 by a Seed-bearing Flora. Plants thus finally broke away from the 

 physiological tie to external water for their fertilisation. Seed- 

 production is carried on in a compact Strobilus, or Flower. Evidence 

 of the steps of segregation of the " general purposes shoot " into dis- 

 tinct nutritive and propagative regions may still be traced in favour- 

 able cases (see p. 241). These regions once established took each its 

 own independent line of specialisation in the evolution of Seed-bearing 

 Plants. The vegetative region which appears first in the individual 

 life commonly develops normal foliage ; but under special conditions 

 it may become xerophytic, scandent, parasitic, or saprophytic, suiting 



