60 



MORPHOLOGICAL DEVELOPMENT. 



and a channel, being here unrestrained by the early-formed 

 fronds folded round it, goes on without the bursting of these. 

 Hence arises a leading character of what is called exogenous 

 growth — a growth which is, however, still habitually accom- 

 panied by exfoliation, in flakes, of the outermost layer, con- 

 tinually being cracked and split by the accumulation of 

 layers within it. And now if we examine plants 



of the exogenous type, we find among them many displaying 

 the stages of this metamorphosis. In Fig. 95, is shown a 

 form in which the continuity of the axis with the mid- rib of 

 the leaf, is manifest — a continuity that is conspicuous in the 

 common thistle. Here the foliar expansion, running some 

 distance down the axis, makes the included portion of the 



axis a part of its mid-rib ; just as in the ideal types above 

 drawn. By the greater growth of the internodes, which are 

 very variable, not only in different plants but in the same 

 plant, there results a modification like that delineated in 

 Fig. 96. And then, in such forms as Fig. 97, there is shown 

 the arrangement that arises when, by more rapid develop- 

 ment of the proximal portion of the mid-rib, the distal part 

 of the foliar surface is separated from the part which em- 

 braces the axis : the wings of the mid- rib still serving, how- 

 ever, to connect the two portions of the foliar surface. Such 

 a separation is, as pointed out in § 188, an habitual occur- 

 rence ; and in some compound leaves, an actual tearing of the 

 inter- veinous tissue, is caused by extra growth of the mid-rib. 

 Modifications like this, and the further one in Fig. 98, we 

 may expect to be established by survival of the fittest, among 



