82 



ED >NGATU >\ < IF THE STEM 



with the exception of a few small spiral vessels that are formed 

 as the first cells of the vascular bundles. 



The monocotyledons, plants distinguished usually by their 

 parallel veined leaves and single seed leaf, like the palms, lilies, 

 ses, etc., are characterized by stems that do not increase 

 materially in diameter. Growth is largely confined to the top 

 of the stem and consequently it can only elongate, forming a 

 very regular, columnar trunk. The reason of this is apparent 

 when cross sections of such stems are examined I Fig. 45. D). 

 The vascular bundles are more or less scattered throughout the 

 stem and it is especially noteworthy that there is no cambium 



separating the xylem and phloem. It will he seen by examining 



Fig. 'x'. that the tissues of the bundles are of a similar character 

 and have the same arrangement as in the dicotyledons ( Fig. 41) 



save for the absence of the cambium which prevents the addition 



of new cells to the xylem and phloem. Consequently there can 

 be no considerable increase in the diameter of the stem and no 

 necessity therefore for the protective layer of cork cells. 



e c x * 



Fig, 61. Longitudinal Bection of the tip of a growing stem: .. epidermis 

 ding over surface of the entire tip; a, formative region; b, upper 

 portion of the rone of elongation; c, cortex; x, cells of the central 1 

 that by further growth form the vascular bundles; /, first appearai 

 the le 



41. Apical Growth of the Stem. — All stems are characterized 

 by a more or less extended apical growth. The changes pro- 

 duced in stems by the growth of the cambium onlj result in 

 increasing their diameters bul the elongation of all stems is 



