MORPHOLOGY OF HIGHER PLANTS. 



209 



being regarded as a modification of those of the closed collateral 

 tj'pe. Where increase in thickness of the stem takes place, the 

 cells of the endodermis are altered in shape, and the walls of the 

 hypodermal cells become thickened with lignocellulose and suberin. 

 The stem as well as the root develops abnormal and anomalous, 

 structures, although they are not of sufficient importance to war- 

 rant consideration in this connection. 



? S 



^'r 



Fig. 116. Section of a four-year-old stem of a pine cut in winter; q, view in trans- 

 verse section; 1, radial-longitudinaj section; t, tangential-longitudinal section; f, spring 

 wood; s, fall wood; m, pith; i, 2, 3, 4, successive years' rings of growth in which i shows 

 the dividing line; ms, medullary rays in transverse section; ms^, ms^^, medullary rays 

 in radial-longitudinal section; ms^", medullary rays in tangential-longitudinal section; 

 o, cambium; b, bast; h, resin-canals; br, bork. — After Strasburger. 



STRUCTURE OF THE LEAF. 



The leaf usually consists of three kinds of tissues (Fig. 117) : 

 (i) epidermal, (2,) parenchymatous, and (3) fibrovascular ; in 

 some cases there may also be a development of collenchyma. 



The Epidermis forms the surface of the leaf and may con- 

 sist of one or more layers of cells. The outer walls are cutinized, 

 and when nearly smooth the leaf is said to be glabrous. They 



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