DIV. I 



MORPHOLOGY 



103 



When a sclerenchymatous sheath surrounds a collateral bundle it is frequently 

 interrupted at the sides, opposite the junction of the xylem and phloem, by 

 parenchymatous or less thickened and lignified elements. These long strips 

 facilitate the exchange of water and nutritive substances between the bundle and 

 the surrounding tissues. 



In order to understand the construction of the vascular bundles 

 and the differences between the various types their ontogenetic 

 development must be taken into consideration. The primary vascular 

 bundles are developed from strands of elongated meristematic cells. 





ft 1 



o 





FIG. 120 B. Longitudinal section of a vascular bundle from the stem of Zea Mais, a and a', Rings 

 of an annular tracheide ; v, sieve-tubes ; s, companion cells ; cp, protophloem ; I, intercellular 

 passage ; vg, sheath ; sp, spiral tracheides. ( x ISO. After STRASBUROER.) 



In these the differentiation of the tissues proceeds gradually over a 

 period of time. So long as the portion of the plant is still growing 

 actively in length the main portion of the strand of meristem remains 

 undifferentiated. Only at limited regions of the strand, usually at 

 the outer and inner margins, are a few elements transformed into 

 permanent tissue. These elements, which are suited to undergo 

 stretching, are on the one hand annular and spiral tracheides, and on 

 the other sieve-tubes with or without companion cells. They form 

 the protoxylem and protophloem respectively. Only when growth 

 in length is finished do the bundles become fully differentiated, the 

 differentiation proceeding from the protoxylem and protophloem. In 



