Foliar Ray in the Wood of the Dicotyledons. 649 



Fig. 15 the ten sheets of ray tissue are distributed in five pairs of approxi- 

 mated rays. This approximation of the lateral leaf-trace rays enables 

 their concentrated retarding influence upon growth to depress the included 

 segment of tracheal tissue below the general contour of the stem (PL LXII, 

 Fig. 3, and PI. LXIII, Figs. 13, 15). In addition to the lateral leaf-trace rays 

 foliar rays are developed in oak and other plants in relation to the median 

 traces of the leaf. However, m oak these rays are in most cases less strongly 

 developed in the young twig than are the rays related to the lateral traces. 



6. The origin and development of foliar rays show clearly that the 

 statements of Sanio, Sachs, and de Bary, pointing to the origin of large 

 rays as inclusions of fundamental tissue or ground parenchyma between 

 putative fibro- vascular bundles and the development of so-called fascicular 

 and interfascicular segments from supposed fascicular and interfascicular 

 cambiums, lead to extremely misleading conclusions. 



7. All segments of the stem are essentially ' fascicular ', since in the 

 seedling plant of primitive Dicotyledons the stele is an undivided tubular 

 cylinder without indication of the putative fascicular and interfascicular 

 segments and large rays. 



8. The development, in relation to the traces of the leaves, of large 

 sheets of storage tissue from congeries of uniseriate rays has a strong 

 dissecting effect upon the stele, producing the supposed fascicular and inter- 

 fascicular segments of oak, vines, and semi-herbaceous plants. 



Eames (5), in an investigation of herbaceous and semi-herbaceous Angio- 

 sperms, secured interesting evidence which indicates strongly that herbaceous 

 Angiosperms have been derived from forms which possessed strongly 

 developed secondary growth. In the life-history of many plants there 

 occurs a transition from a tubular stele, in younger portions of the plant, 

 to a ring of separate fibre-vascular bundles in subsequently formed parts 

 of the stem. For example, the prostrate biennial or perennial stems of 

 Potentilla palustris, Scop., as well as the seedling plant, possess an unbroken 

 central cylinder, whereas the cylinder of the erect annual stem a short dis- 

 tance above the rhizome breaks down into a typically herbaceous ring of 

 separate bundles. The herbaceous type of central cylinder has resulted, 

 therefore, from the reduction in size of Dicotyledonous plants in later 

 geological periods, coupled with the evolution of foliar rays whose dissecting 

 effect has been progressively increased. 



More recently Professor Groom (6) has published the results of an 

 investigation upon the annual ring and medullary rays of Quercus. There 

 are certain fundamental objections to the conclusion reached by Professor 

 Groom, that ' it is impossible at the present to decide whether in Quercus the 

 broad-rayed or the narrow-rayed type was primitive '. 



In studying lines of evolutionary modification in plant structures it is 

 essential not only that a careful study be made of the comparative anatomy 



