THE EVOLUTION OF HERBACEOUS PLANTS 291 



however, cannot be regarded as entirely conclusive, since the delicate 

 leaves of herbs are probably less readily preserved than are the 

 tougher ones of most trees and shrubs. Among the lower orders, 

 however, it is quite certain that the ancient fossil lepidodendrids, 

 sigillarians, and calamites, for example, were woody plants, and 

 that their modern representatives are all herbaceous. 



From anatomy. — The most important anatomical difference 

 between a woody plant and an herb lies in the degree of activity 

 of the cambium. In the former group this tissue lays down a 

 thick ring of wood which increases in size year by year. In the 

 latter the woody ring is either very much thinner or, more fre- 

 quently, is broken up into separate bundles. In such cases the 

 so-called 'interfascicular" cambium, opposite the gaps between the 

 bundles, is much reduced or quite inactive; and in the bundles 

 themselves the cambium is often merely vestigial. That such a 

 discontinuous cambium is not the primitive type is shown by the 

 fact that in various ancient groups which were just acquiring 

 secondary growth (as shown by the structure of their fossils) the 

 cambium always began as a perfectly continuous ring and did not 

 arise from the union of isolated fascicular cambia. 



Furthermore, in the vascular tissue of most herbs, medullary 

 rays and wood parenchyma are poorly developed or absent. These 

 structures, however, always characterize the more primitive types 

 of angiosperm wood. 



These two pieces of evidence point to the conclusion that the 

 herbaceous stem has been reduced from a primitive woody one. 

 This reduction has been accomplished by a marked decrease in the 

 activity of the cambium, and in many instances also by an increase 

 in the height and width of certain of the medullary rays, resulting 

 in the breaking-up of the continuous woody ring into a series of 

 bundles. In its general topography the stem of an herb resembles 

 that of the young twig of its woody relatives. 



From phytogeny. — It is generally admitted that the angiosperms 

 have been derived either from the Bennettitales or from some stock 

 derived from the Coniferales. There are no herbaceous forms in 

 either of these groups. Artiong living angiosperms it is uncertain 

 whether the nearly naked flowered Amentiferae or the complete 



