THE DICOTYLEDONOUS STEM 59 



fibers of the outer rind and the frequent fibre-vascular bundles 

 just within this, are arranged in the best way to secure stiffness. 

 In a general way, then, we may say that the pith, the bundles, 

 and the sclereiichymatous rind are what they are and where 

 they are to serve important mechanical purposes. But they have 

 other uses fully as important (see Chapter vin). 



73. Growth, of monocotyledonous stems in thickness. In 

 most woody monocotyledonous stems, for a reason which will 

 be explained later in this chapter, the increase in thickness is 

 strictly limited. Such stems, therefore, as in many palms and 

 in rattans, are less conical and more cylindrical than the trunks 

 of ordinary trees, and are also more slender in proportion to 

 their height. 



STEM OF DICOTYLEDONOUS PLANTS 



74. External characters. It is not easy to make any gen- 

 eral statements about the external characters of dicotyledonous 

 stems, on account of their very great variety of form. The stu- 

 dent in his examination of twigs in connection with Chapter VI 

 has learned a little about the appearance of a few woody stems. 

 In general, the nodes are much less marked than in stems of 

 corn, bamboo, and other grass-like forms. In the case of decid- 

 uous-leaved dicotyledonous plants, the scars left by fallen leaves 

 are characteristic, quite unlike those mentioned in Sec. 70. 



75. Internal structure. 1 If one begins his study of the struc- 

 ture of dicotyledonous stems with the one-year-old stem of a 

 woody plant or with the stem of some such robust annual as 

 hemp, sunflower, or the great ragweed, he will find it to be com- 

 posed of a somewhat cylindrical pith, surrounded by a layer 

 of wood usually of pretty even thickness, which is in its turn 

 surrounded by a layer of bark (Fig. 56). 2 



1 For an account of the structure of the pine stem, see Sec. 352. 



2 Of course these layers are nearly cylindrical tubes, filled by pith or by 

 wood and pith respectively. They are not of perfectly circular cross section, 

 and they taper somewhat. 



