348 THE STRUCTURE OF ECONOMIC PLANTS 



with an abundant vascular supply. Each strand is surrounded by an 

 endodermis which is continuous with that of the root, and which is 

 not evident quite so far as the point at which the tissues merge into 

 the meristem." 



There is no cambial activity and secondary growth in the devel- 

 opment of the nodule, and the primary growth is limited so that 

 it soon attains full size. Division may take place in the large cen- 

 trally located cells which contain the bacteria, and intercellular 

 spaces usually are formed. Under normal conditions, the relation 

 of the bacillus to the host plant is a symbiotic one and there is no 

 destruction of the central cells until late in the ontogeny of the host 

 plant; but Nemec, as well as Brenchley and Thornton, points out 

 that under unfavorable conditions there may be a complete disin- 

 tegration of the central parenchymatous cells, resulting in large 

 intercellular spaces and a general degeneration of the bacteria- 

 containing tissue. Where there is a nutritional deficiency in the 

 soil, such as the absence of boron, there may be a development of 

 incipient nodular tissue in the cortex only, and an absence of peri- 

 cyclic activity or the development of tracheids in the nodules. 



In the symbiotic relationship, the nitrogen bacteria receive car- 

 bohydrate material from the host plant and contribute nitrogenous 

 compounds to the host, but the bacteria may become parasitic when 

 the carbohydrate supply to the nodule is suspended. Under these 

 conditions, the bacteria attack and destroy the cytoplasm of the 

 cells in which they are located, ultimately destroying the cell walls 

 as well; and, similarly, a destruction of the nodule tissue by bac- 

 teria occurs in old nodules at the end of the growing season. 



Vascular Transition in the Axis. — The root-stem transition 

 is not completed in the short hypocotyl, but also involves the first 

 three internodes of the stem so that the stele is not an endarch 

 dictyostele until the fourth internode is reached. This situation has 

 been investigated by Dangeard (8), Herail (15), Tourneux (2.7), 

 Compton (5), Gourley (11), and the following account is based in 

 part on these reports, supplemented by further investigation of a 

 confirmatory nature.^ 



' A recent memoir (MuUer, C, '"La tige feuillee et les cotyledons des Viciees a germination 

 hypogee." LaCellule 46: 195-354, 1937) has just been received. This includes a comparative 

 study of the vascular anatomy of the seedlings of five representatives of the tribe Vicieae: 

 Pisum sativum, Vicia sativa, V. Faba, Lens esculenta, and Cicer arietinum. In this extensive 

 study, Muller describes the vascular anatomy of the seedling in each instance. The author 

 presents a somewhat different interpretation of the relation of the vascular structures than 



