AMPLIFICATION OF THE SPOROPHYTE 247 



It is possible to conceive of an indefinite increase of the sporophyte, 

 by continued cell-division and progressive sterilisation, in a body main- 

 taining a simple form ; but mechanical and physiological checks impose 

 a moderate limit. The Bryophytes illustrate in some of their forms such 

 progress in the sporophyte successfully carried out to a relatively high 

 degree of complexity. But in all their more advanced types there is a 

 distinction of apex and base, the basal region being sterile and the apical 

 region fertile. Their sporogonia, however, always retain a simple form, 

 and with few exceptions the radial type of construction : they are all 

 alike also in having a single continuous spore-sac. This is plainly a type 

 of construction which has its limits imposed by mechanical and physio- 

 logical conditions. Reasons such as these have tended to prevent the 

 Bryophytes from developing their sporogonia beyond a very moderate 

 size. They show, however, very clearly on comparison the successive 

 steps by which progressive sterilisation may advance the complexity of 

 a simple type of sporophyte (Chapters III. and IX.). 



But the Vascular Plants, while showing the same plan of life-cycle, 

 have been able to continue development without those mechanical and 

 physiological checks operating upon their spore-output. The outstanding 

 features in which they are more free than the Bryophytes follow from 

 the segregation of sporogenous tissue in distinct sporangia, and the 

 formation of appendicular organs. The biological advantages thus attained 

 are obvious : a plurality of sporangia makes possible the separate, and 

 more efficient nutrition of each : thereby also the mechanical difficulties, 

 which act in limiting the Bryophyte sporogonium, are effectively avoided. 

 On the other hand, the development of appendicular organs makes 

 independent self-nutrition of the sporophyte really effective, while the 

 position of the sporangia on the appendages facilitates the dispersal of 

 the spores. The palaeontological record shows conclusively that both of 

 these features were of very early date, and their consequences are 

 illustrated in the earliest fossils of which there is any detailed knowledge 

 (Chapter XVIII.). The advantages secured by an unrestricted type of 

 development were doubtless such as to lead to a rapid advance. It 

 can therefore be no matter for surprise that connecting links between 

 the two states are absent, even supposing the two phyla, in which they 

 are characteristically shown, to have had some degree of community of 

 origin. 



The Pteridophytes show diversity of type, according to the size of 

 their appendages : those which are smaller-leaved, as in the Lycopods, 

 Equiseta, and Sphenophylls, have as a rule a terminal strobiloid fructifica- 

 tion, though this is not always clearly differentiated from the vegetative 

 region. In the Fern-like types the fructification is disposed more 

 generally over the enlarged leaves. As in the Bryophyta so in the 

 strobiloid Vascular Plants, a sterile basal region precedes the terminal 

 fertile strobilus. This vegetative region may be held to be a phase 



