450 



those peculiar grape-like clusters of cell substance incrusted with calcium (c) 

 known as cystoliths. 



The close structure of the upper epidermis of the leaf must prevent the 

 passage of air, while the lower epidermis is well fitted for this purpose. 

 The spong}^ parenchyma shows large intercellular spaces (i), the enclosed 

 air in which can pass out through the air chambers under the stomata (a) 

 and the stomata (st) to the outside, making room for the freshly entering 

 outer air. The conduction of water takes place through the leaf veins, one 

 of which is seen in section at g and shows at r the large ducts. The course 





Fig-. 87. Cros.s-section througi 



of the rublKM' 



of organized building substances, produced in the leaf and flowing down 

 toward the trunk, is shown at sch, the sheath of the vascular bundle; k 

 indicates the place at which the cells begin to enlarge because of an excess- 

 ively increased turgor, thus filling the intercellular spaces and forming, 

 therefore, first of all, "internal intumescences." The excessive water con- 

 tent manifests itself still more in the peripheral tissue, since, exposed only 

 to the pressure of the epidermis, its cells elongate into tubes and, together 

 with the epidermis, can be pushed outward (int). 



Actually, therefore, the tubercle disease of the rubber plant is a regular 

 intumescence which belongs to the previous division. We have, however, 



