354 Haeeis ]M. Benedict 



In other words, it must be a visible effect of a progressive senile change 

 occurring in the meristematic cells which produce the leaf. 



An examination of the vein islets under the microscope show^s that a 

 majority of the photosjiithetic cells are not in direct contact with the 

 veinlets. Such cells must get then- water and inorganic nutrients by 

 osmosis and diffusion thi'ough the cells which separate them from the 

 veinlets, .and all outgoing materials must also pass thi'ough the enclosing 

 cells. The larger the vein islet, therefore, the greater wUl be the average 

 distance through w^hich materials must be conducted before the}^ reach 

 the veinlets or the cells. It is obvious that the maintenance of efficient 

 ingress and egi'ess is vital to the hfe of the cells and the hfe of the plant. 

 Since most of the ceUs are dependent on the abHity of smTounding ceUs 

 to conduct materials, the process of diffusion thi'ough photosynthetic 

 cells is of vital importance to the whole plant. 



The results of Askenasy (1870), Volkens (1884), Massart (1902), and 

 Schuster (1908) indicate that if the ceUs of the leaves are suffering for 

 water additional veinlets are produced. In other words, the develop- 

 ment of veinlets is in part controlled by the condition of the cells suppUed 

 by them. If the photosynthetic cells are suffering from the lack of some- 

 thing which the veinlets can supply, the effect is to stimulate the develop- 

 ment of additional veinlets. An interesting theory of the origin of the 

 small blood vessels in the developing animal embryo, advanced by Miihl- 

 mann (1900), is based essentiaUj^ on the same idea. As each aggregate 

 of cells in a developing embiyo increases in size, its central cells wiU be 

 pushed farther away from the blood vessels at its margm. Finally this 

 distance becomes so gi'eat that osmosis and diffusion through the inter- 

 vening cells cannot efficiently provide for the needs of the central cells. 

 This lack of efficient nutrition and excretion causes certain modifications 

 in the cellular activities. These specific modifications stimulate the forma- 

 tion of branches from the origuial blood vessels, which penetrate the ceU 

 aggregate. The entrance of the blood vessel is physiologically the 

 equivalent of a reduction in the size of the aggregate. 



In the leaf of the young plant, the vein islets are as large as will permit 

 efficient conduction to and from the central cells. There is no further 

 increase in size. But if the protoplasm of the cells of the aggregates 

 changes with age in such a way that osmosis and diffusion through them 

 are being rendered increasingly difficult, the phj^siological effect will be 



