RECENT WORK, ETC. 465 



if we remember that he started with the consideration of 

 the root. Apart from more special researches, his first 

 comprehensive work on general anatomy was devoted to 

 this organ (4), and to that work our knowledge of the 

 comparative structure of the root (so wonderfully uniform 

 in its main features) in the various classes of vascular 

 plants, is very largely due. The work in question, though 

 intended to treat generally of the "symmetry of structure" 

 in vascular plants, never advanced beyond the root, a fact 

 which invited criticism. It has, however, borne abundant 

 fruit in later years. Broad views on anatomical questions 

 are likely to be attained by those who begin with the 

 investigation of entire organs, rather than of special forms 

 of tissue, and the fact that the organ chosen by Van 

 Tieohem was one in which the svstems of tissue stand out 

 more clearly than in any other was fortunate for botany. 

 In his Treatise on Botany (5), Van Tieghem keeps to the 

 same order, and describes the structure of the root before 

 that of the stem and leaf. 



The characteristic feature of the anatomical teaching of 

 Van Tieghem and his school, is the recognition of the 

 central cylinder or stele , as a definite region comparable to 

 cortex and epidermis. In the root the arrangement is 

 clear enough. The central cylinder is perfectly well defined, 

 including within it the ring of alternating bundles of wood 

 and bast, accompanied by a certain amount of parenchyma 

 (conjunctive tissue), the outer layer of which constitutes the 

 pericycle, or limiting layer of the stele, so important from 

 its various generative activities. Surrounding this is the 

 cortex, the inner layer of which is the endodermis, and on 

 the outside of the cortex there is the piliferous layer, which 

 however is not in all cases recognised by Van Tieghem as 

 a morphologically distinct epidermis. 



Now the French botanist applies just the same prin- 

 ciples to the stem. Here also he distinguishes three 

 regions, central cylinder, cortex, and epidermis (5, p. 737). 

 The cylinder is made up of the vascular bundles, and of 

 conjunctive tissue. The pith is the part of the conjunctive 

 tissue which lies within the bundles, that lying between 



