134 



A TEXTBOOK OF BOTANY 



[Ch. IV, 3 



Some stems present special cellular features, of which the 

 most striking is the latex system. This latex, the milky 

 juice of plants which contains so many diverse substances 

 of uncertain significance (page 108), is found mostly in long, 

 slender, closed tubes branching irregularly through the 



softer tissues, and even the 

 wood of stems, leaves and 

 other parts (Fig. 88). 



The Aristolochia, and 

 other stems just men- 

 tioned, are all exogenous. 

 The endogenous type pre- 

 sents some, though no 

 great, cellular differences. 

 Thus, as exemplified in the 

 Corn (Fig. 89), the bundles, 

 of course lacking cambium, 

 present each a very distinct 

 strand of thin- walled, regu- 

 larly-arranged phloem, 

 alongside of which is the 

 strand of xylem, distin- 

 guished by very large ducts 

 and commonly a great air 

 passage; while the apparent bundle-sheath encircling the 

 bundle has been found to develop by extension from the 

 xylem. In such stems there is no distinction of pith, 

 medullary rays, and cortex; but all are merged together 

 in one pith- like general ground tissue (Fig. 74) . 



In the foregoing description of the structure of stems, we 

 have considered only one type of fibro-vascular bundle, — 

 the kind having parallel strands of phloem and xylem. 

 Many other types and sub-types, however, occur, as well as 

 many special forms of arrangement of the bundles within 

 the cylinders and in relation to the leaves. It has recently 

 been found that these morphological features of stem 



Fig. 88. — The latex system in Lac- 

 tuca virosa, in section ; X 180. (From 

 Kerner.) 



