THE PLANT BODY 11 



leaves of the woody Ranales. The pattern of minor vein arrangement 

 is also of two major types — reticulate or netted, and parallel or striate — 

 but there are intermediate types. There are many exceptions to the 

 characterization of the leaf venation of dicotyledons as netted and 

 that of the monocotyledons as parallel; the leaves of many of the lower 

 monocotyledons have netted veins, and some of the dicotyledons — the 

 Epacridaceae, for example — have parallel veins. The venation of the 

 Epacridaceae contrasts strongly with that of the closely related Ericaceae 

 and has clearly been derived from the reticulate type. (Modification of 

 nodal structure has accompanied the change in venation; a multilacunar 

 node in the Epacridaceae has replaced the unilacunar node in the 

 Ericaceae. ) 



Perhaps the most important feature in the arrangement of the lesser 

 veins is the presence of free vein endings within the vein eyelets in some 

 taxa and the absence of free endings in other taxa. The view that signif- 

 icant changes in size of the vein eyelets occur with increasing age of the 

 plant was shown to be without foundation. Vein eyelets are usually of 

 irregular form, but, in some tropical genera, tliey are rectangular and 

 remarkably uniform in size. 



The petiole arises late in the ontogeny of the leaf, by intercalary 

 growth of a region at the base of the lamina. Increase in length is 

 probably chiefly by cell enlargement. The delayed development of the 

 petiole has been considered evidence that the petiole is a recent de- 

 velopment in the specialization of the leaf. But this seems unlikely, 

 because sessile leaves can often be shown, by comparison with the 

 leaves of related taxa, to have lost their petioles. The leaves of the primi- 

 tive woody dicotyledons are petiolate, and the leaflike carpels of many 

 of them are stipitate. (The homology of leaf and carpel has been 

 strongly supported in recent years by evidence from ontogeny and 

 anatomy. ) 



Stipules are lateral parts or appendages of the leaf, usually borne one 

 on each side at, or near, the base of the leaf. When borne on the stem 

 free from the leaf itself, they appear to be independent organs and 

 have been called "cauline stipules," an unfortunate term which implies 

 that they are part of the stem. But anatomical structure shows that 

 these stipules are a part of the leaf; their vascular supply is derived 

 from the lateral leaf traces. Although this anatomical relationship was 

 shown in 1880, its morphological significance was not recognized until 

 nodal anatomy was critically studied in the early decades of the 

 twentieth century. 



Stipules have been called "appendages of the leafbase, not of the leaf" 

 because they "arise from the leafbase." But only independently at- 

 tached stipules arise in this manner; other stipules arise congenitally 



