132 BOTANY part i 



termed mesopliyll), bounded by an epidermis and traversed by 

 vascular bundles. Sheaths are present around the bundles, extend- 

 ing to their fine terminations. The cells composing these mesophyll 

 sheaths are as a rule elongated and not separated by intercellular 

 spaces. Besides limiting the vascular bundles from the mesophyll, 

 the sheaths perform the important function of conducting soluble 

 carbohydrates from the leaf to the stem. The larger vascular 

 bundles are usually accompanied by strands of sclerenchyma ; these 

 disappear from the finer branches. Other strands of sclerenchyma 

 not connected with the vascular bundles may also occur in the 

 mesophyll and contribute to the rigidity of the leaf. 



The mesophyll passes into the primary cortex of the stem, while the vascular 

 bundles are continuous with the central cylinder. Thus in the leaf, tissues corre- 

 sponding to the cortex and central cylinder of the stem remain distinct from one 

 another. 



The mesophyll of the coloured flokal leaves of the Angiosperms 

 usually consists of a somewhat loose tissue, containing intercellular 

 spaces and traversed by vascular bundles. The laminae of many 

 assimilating foliage leaves, especially of shade-loving plants, may 

 have a similar uniform structure ; but they are usually more com- 

 plicated, and exhibit a difference in the structure of their upper and 

 lower sides (Fig. 142). In such dorsiventral structures the upper 

 epidermis (Fig. 142, ep) is succeeded by one or more layers of 

 cylindrical parenchymatous elements elongated at right angles to the 

 surface, and known as the PALISADE cells (jil). These are especially 

 rich in chlorophyll, and are often separated laterally from one another, 

 so that the conduction of substances is limited to the direction of 

 their longer axis. Adjoining the palisade parenchyma, and extending 

 to the epidermis {ep") on the under suiface of the leaf, is a loose tissue 

 called the spongy parenchy]\ia {sp). In contrast to the palisade 

 cells, the cells of the spongy parenchyma are less abundantly supplied 

 with chlorophyll ; they are also much more irregular in shajje, and 

 have large intercellular air-spaces between them. The palisade cells 

 are elongated in the direction in which the rays of light penetrate 

 the leaf-lamina, and by this means are particularly adapted to their 

 special function of assimilation. The spongy parenchyma, on the other 

 hand, is arranged to facilitate the free passage of gases, and to that 

 end develops large intercellular spaces in direct communication with 

 the stomata of the lower epidermis. HabePkLANDT (^^^) has estimated 

 that to every square millimetre of surface in a leaf of liic'mus communis 

 there are, in the palisade cells, 403,200 chlorophyll granules; in the 

 cells of the spongy parenchyma only 92,000 ; that is, 82 per cent of 

 all the chlorophyll granules belong to the upper surface of the leaf, 

 and only 18 per cent to the under side. The palisade cells are often 

 arranged in groups, in which the lower ends of the cells of each group 



