24 



THE AMERICAN BOTANIST 



fact the hepatica belongs to a comparatively primitive familv 

 of plants in which the number and arrangement of the floral 

 parts are by no means firmly fixed and in which, therefore, 

 variations from the exact flower formulas of other families are 

 to be expected. 



Evolution and Leaf Form. — Even the botanizer is usu- 

 ally well informed as to differences in leaf venation, but it may 

 never have occurred to him to inquire how the various types 

 came to be and how they are related to one another. E. W. 

 Sinnott and I. Bailey, after a study of the vegetation of the 

 world, have come to the conclusion that the leaves of primitive 

 plants were palmately veined and that the pinnately branched, 

 lobed and veined leaves so numerous at present are modified 

 forms of the earlier type. In many of our plants with noniially 

 pinnately veined leaves, we may find extra large leaves revert- 

 ing to the primitive character in the matter of veining or the 

 base may be palmately veined and the apex pinnate. By the 

 lengthening of a palmately veined leaf the pinnate form could 

 easily have been brought about. There are also numerous 

 oblong or narrow leaves like those of the common plantain in 

 which the veining is essentially palmate though the shape of 

 the leaf does not indicate it and the veins, instead of spreading 

 out are all brought together at the tip. Palmately veined leaves, 

 of this type, however, are supposed to have given rise to the 

 parallel veined leaves of monocots. It is curious to note in thib 

 connection that the cotyledons of many dicotyledonous plants 

 have a system of veinhig that is essentially like that of the 

 monocots. At present there are few tropical species with 

 palmately veined leaves. ]\Iost of those still left to us are found 

 in temperate regions. They are quite numerous in the rose and 

 saxifrage families but even in these families, the tropical species 

 are almost certain tO' be pinnately veined. From considerations 

 of these peculiarities of leaf veining, it is often assumed that the 

 earliest plants were woody and that herbs have originated from 

 such ancestors. 



