42 MORPHOLOGICAL DEVELOPMENT. 



unit called a leaf, has, in each higher plant, a typical form, 

 due to the special arrangement of these cells around a mid- 

 rib and veins. If the multiplication of morphological units, 

 at the time when the leaf -bud is taking on its main outlines, 

 exceeds a certain limit, these units begin to arrange them- 

 selves round secondary centres, or lines of growth, in such 

 ways as to repeat, in part or wholly, the typical form: the 

 larger veins become transformed into imperfect mid-ribs of 

 partially independent leaves; or into complete mid-ribs of 

 quite separate leaves. And as there goes on this transition 

 from a single aggregate of cells to a group of such aggregates, 

 there simultaneously arises, by similarly insensible steps, a 

 distinct structure which supports the several aggregates thus 

 produced, and unites them into a compound aggregate. These 

 phenomena should be carefully studied; since they give us a 

 key to more involved phenomena.* 



§ 189. Thus far we have dealt with leaves ordinarily so 

 called: briefly indicating the homologies between the parts 

 of the simple and the compound. Let us now turn to the 

 homologies among foliar organs in general. These have been 



* There is much force in the criticism passed on the above paragraph, and 

 by implication on some preceding paragraphs, that though in plants which 

 tend to produce compound leaves the production is largely dependent on the 

 supply of nutriment, yet the unqualified statement of this relation as a gen- 

 eral one, is negatived by the existence of plants which bear only simple 

 leaves, however much high nutrition causes growth. But mostly valid though 

 this objection is, it is probably not universally valid. I am led to say this by 

 what occasionally occurs in flowers. The flowering stem of the Hyacinth is 

 single; but I have seen a cultivated Hyacinth in which one of the flowers 

 had developed into a lateral spike. Still more striking evidence was once 

 supplied to me by Agrimony. All samples of this plant previously seen had 

 single flowering spikes, but some years ago I met with one, extremely luxuri- 

 ant, in which some flowers of the primitive spike were replaced by lateral 

 spikes ; and I am not sure that some of these, again, did not bear lateral 

 spikes. Now if in plants which, in probably millions of cases, have their 

 flowering stems single, excessive nutrition changes certain of their flowers into 

 new spikes, it is a reasonable supposition that in like manner plants which 

 are thought invariably to bear only single leaves, will, under kindred condi- 

 tions, bear compound leaves. 



