60 



MORPHOLOGICAL DEYELOrMENT. 



and a diannel, being here unrestrained by the early-foniied 

 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 t}^e, we find among them many displaying 

 the stages of this metamorphosis. In Fig. 95, is showTi 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 

 dra^sTi. By the greater growth of the internodes, which are 

 very variable, not only in diflej'ent 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 tlie 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 b}^ survival of the fittest, anion p 



