CHAP, ii The Differentiation of the Tissues 215 



all put forward the view that the anatomical differences 

 are due to and regulated by the transpiration current. 



The chief anomalies in the thickening of woody trunks 

 had been discovered prior to 1860. A few additions were 

 made to our knowledge afterwards. In 1864 Sanio dis- 

 covered the peculiar occurrence, in the stem of Tecoma 

 radicans, of a second cambium ring, situated within the 

 circle of the wood bundles, and found that it produces 

 bast and wood in order exactly contrary to the order of the 

 normal ring. 



In 1874 Schmitz found in the rhizome of Rheum the 

 occurrence of several partial cambiums, producing again 

 inverted orientation of their products. 



The details of the formation of periderms by phellogen 

 were critically investigated by Sanio in 1860. To him are 

 due the foundations of our modern knowledge of the histo- 

 logical facts. 



It will be seen that different views of the construction 

 of the stele or steles are possible according to the inter- 

 pretation that is placed upon the endodermis. Herein may 

 be said to lie a certain weakness of the system, for the 

 endodermis is rather a physiological than a morphological 

 structure. True, as originally propounded by De Bary, it 

 was intended to designate the inner boundary of the cortical 

 area. But certain histological features were combined with 

 this topographical limitation, and by many they came to 

 be regarded as more truly characteristic than the latter. 

 When the cases of polystely and astely came to be examined, 

 difficulty arose in distinguishing the limits of a stele which 

 in many cases seemed to be identical with a fibro-vascular 

 bundle, and if diagnosed by the presence of a layer showing 

 these endodermal histological features, was sometimes repre- 

 sented by nothing more than a mere strand of parenchyma, 

 as shown by Pfitzer in certain species of Equisetum. 



The possession of these particular thickenings by the 



