STEMS, ROOTS, AND LEAVES 



109 



cambium layer, so that 

 such stems have no 

 power of increasing the 

 diameter of the stem 

 by cambial growth. 

 There is no distinctive 

 pith, since the bundles 

 from the leaves arch 

 into the center of the 

 stem and occupy por- 

 tions of the pith region, 

 as indicated in the fig- 

 ure. In many mono- 

 cotyledons the cortex 

 is also difficult to dis- 

 tinguish, since the vas- 

 cular bundles occur in 

 it and no sharp division 

 line exists between cor- 

 tex and pith. In these 

 unusual stem types the 

 storage, mechanical, and 

 conducting functions 

 are provided for in 

 much the same general 

 way as in dicotyledons, 

 except that the tis- 

 sues are differently dis- 

 posed in the stem and 

 in the bundles. The 

 structure of the stem 

 in the seedling of a 

 monocotyledon often 

 resembles that of the 

 dicotyledons, so that 

 botanists are agreed 

 that the monocotyle- 

 don ous type of stem 

 originated from the 

 dicotyledonous type. 



B 

 FIG. 57. Structure of the stem of corn (Zea mays) 



A, gross structure of the corn stem: c, epidermis; 

 fv, vascular bundles ; p,pith; B, microscopic structure 

 of a vascular bundle : /, parenchyma ; pr, early phloem 

 (protophloem) ; s, sieve tubes ; com, companion cells ; 

 v, large water ducts; sp, spiral duct; a, annular duct; 

 i, intercellular passage 



