THE MOPPHOLOGICAL COMPOSITION OF PLANTS. 33 



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 themselves round 

 secondary centres, or lines of growth, in such ways as to re- 

 peat, in part or wholly, the typical form : the larger veins 

 become transformed into imperfect mid-ribs of partially inde- 

 pendent 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 simul- 

 taneously arises, by similarly-insensible steps, a distinct 

 structure which supports the several aggregates thus pro- 

 duced, 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 homo- 

 logies among foliar organs in general. These have been 

 made familiar to readers of natural history, by popularized 

 outlines of " The Metamorphosis of Plants " a title, by the 

 way, which is far too extensive ; since the phenomena treated 

 of under it, form but a small portion of those it properly in- 

 cludes. 



Passing over certain vague anticipations that have been 

 quoted from ancient writers, and noting only that some 

 clearer recognitions were reached by Joachim Jung, a Ham- 

 burg professor, In the middle of the 17th century ; we come 

 to the Tlieoria Generations, which Wolff published in 1759, 

 and in which he gives a definite form to the conceptions that 

 have since become current. Specifying the views of Wolff, 

 Dr Masters writes, " After speaking of the homologous 

 nature of the leaves, the sepals and petals, an homology 

 consequent on their similarity of structure and identitv of 

 origin, he goes on to state that the ' pericarp is manifestly 

 composed of several leaves, as in the calyx, with this differ- 



