114 BOTANY 



of the stem, they are in reality partial cylinders or schizosteles. In 

 Pteridophytes, the partial cylinders of the leaves join those of the stems, 

 and both have the same structure ; in Phanerogams each partial 

 cylinder of a leaf includes only a single vascular bundle, so that as 

 many partial cylinders as vascular bundles enter the leaf. The meso- 

 phyll and the tissue of the partial cylinders always remain separated 

 in the leaves. The sheaths of fundamental tissue from the central 

 cylinder, which often accompany the vascular bundles when they enter 

 the leaves, eventually disappear with the repeated branchings of the 

 bundles. The mesophyll thereupon forms a mesophyll sheath, which 

 corresponds to the phloeoterma of stems (p. 109), and closes contigu- 

 ously about the free ends of the bundles (Fig. 121). Thus, in the more 

 highly organised plants, the epidermis, primary cortex, and the 



TISSUES OF THE CENTRAL CYLINDER, OR OF THE PARTIAL CYLINDERS, 

 WITH THEIR VASCULAR BUNDLES, FORM ISOLATED TISSUE SYSTEMS, 

 THE MUTUAL INDIVIDUALITY OF WHICH IS MAINTAINED TO THE 



very last ramifications. The cells of the mesophyll sheath are char- 

 acteristically elongated, and are distinguishable by their uninterrupted 

 contact. In addition to the isolation of the mesophyll from the 

 tissue of the partial cylinders, the mesophyll sheath has also to 

 perform the important function of taking up the carbohydrates in 

 solution and of transferring them from the leaf to the stem. The 

 vascular bundles, in turn, provide the leaf with water together with the 

 salts held by it in solution, and also carry away the albuminous sub- 

 stances produced in the leaf. 



The leaf-bundles of Gymnosperms are unbranched, and the necessary communi- 

 cation between the bundles and the surrounding tissue is maintained by means of 

 bundle-flanges. On the vascular side of the bundle, the projecting flanges consist 

 of dead parenchyma without protoplasm, the cells of which contain only water, and 

 are provided with bordered pits, so that in this respect they resemble tracheids ; 

 on the phloem side the parenchymatous cells of the bundle-flanges are full of 

 living protoplasm. The transfusion of the contents of the bundles and the sur- 

 rounding tissue is carried on by means of the bundle-flanges ; the mesophyll receives 

 its supply of water from the vascular portion, while the albuminous substances 

 of the leaf-tissue are in turn transferred to the phloem portion of the bundles. 



In certain families of the Dicotyledons, particularly in the Orassulaceae, the 

 mesophyll of the leaf-lamina forms peculiar masses of tissue called the epitheme 

 between the swollen terminations of the bundles and the epidermis. The cells 

 of the epitheme are small and, for the most part, devoid of chlorophyll ; they 

 are full of water, and joined closely together, leaving only very small interspaces, 

 which are filled with water. The epithemes serve as internal hydathodes (c/. p. 91) 

 for the discharge of water, in most cases by means of water-pores (p. 95) situated 

 immediately over them. 



The mesophyll of the coloured FLORAL leaves of the Angiosperms 

 usually consists of a somewhat loose tissue, containing intercellular 

 spaces and traversed by vascular bundles. The' laminae of many assi- 



