82 



ELONGATION OF 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, 

 grasses, 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 (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 be seen by examining 

 Fig. 60, 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 



FIG. 61. Longitudinal section of the tip of a growing stem : e, epidermis 

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

 portion of the zone of elongation; c, cortex; x, cells of the central region 

 that by further growth form the vascular bundles; I, first appearance of 

 the leaves. 



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 only result in 

 increasing their diameters but the elongation of all stems is 



