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Sutherland and Eastwoods — - The Physiological 
fifty fibro-vascular bundles, one for each ridge, run up through sheath and 
blade. They are of two kinds (Fig. 4 . 1 and 2 ), alternating with one another. 
Round each bundle are two rings of cells. The inner consists of smaller 
cells, regular and very strongly lignified in the larger type, less regular and 
Fig. 4. 1. Transverse section through leaf-blade, showing the distribution of hydathodes, air- 
passages, and sclerenchyma. 2. Portion of 1 very much enlarged : (pa) papilla ; (st) stoma ; 
(sc) sclerenchyma; (hy) hydathode; (ch) chlorenchyma ; (me), motor cell; (c) partition cell; 
(iv) water-storing envelope; <ap) air-passage. 
only slightly thickened in the second ; the other envelope consists of large 
thin-walled elongated cells. In the young plants they contain chloroplasts, 
which, as growth proceeds, disappear first from the upper cells, then from 
those towards the lower surface, and finally from the lateral ones. These 
