MORPHOLOGY 121 



in close contact with one another, but intercellular spaces are present 

 more internally. These intercellular spaces often widen into air- 

 cavities or passages. In many roots a hypoderma giving mechanical 

 support to the epidermis or exodermis is present. The outermost 

 layer of cells of the central cylinder (Figs. 124, 131 p) forms the 

 pericycle, which is also called the pericarnbium ; this is usually a single 

 layer, and in rare cases is wanting. The xylem and phloem portions 

 form separate strands (p. 113), radially disposed and alternating 

 with each other (Figs. 124, 130). It has already been shown that 

 the narrowest elements of the vascular strand are outermost. Roots 

 are described as diarch, triarch, polyarch, according to the number 

 of the vascular strands. For example, the roots of Acorns 

 Calamus (Fig. 124) are octarch, those of Allium Cepa (Fig. 130) 

 hexarch. The vascular strands may eitHer meet in the centre 

 (Fig. 130), or they may surround a central pith (Fig. 124). 



The Leaves are composed of fundamental tissue (which is here 

 termed mesophyll), 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 FLORAL LEAVES of the Angiospei'ms 

 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. 132). In such dorsiventral structures the upper 

 epidermis is succeeded by one or more layers of cylindrical 

 parenchymatous elements elongated at right angles to the surface, and 

 known as the PALISADE CELLS. These are especially rich in chlorophyll, 

 and are often separated laterally from one another, so that the con- 

 duction of substances is limited to the direction of their longer 

 axis (Fig. 132). Adjoining the palisade parenchyma, and extending 

 to the epidermis (ep") on the under surface of the leaf, is a loose 



