PATH OF MOVEMENT. 



47 



is communication throughout the entire length of the stem. (The function of 

 the sieve tubes is supposed to be that for the downward transportation of sub- 

 stances elaborated in the leaves.) 



106. If we section in like manner the stem of the sunflower we shall see simi- 

 lar bundles, but the number is greater than eight. In the garden balsam the 

 number is from four to six in an ordinary stem ^-\mm diameter. Here we 

 can see quite well the origin of the vascular bundle. Between the larger 

 bundles we can see especially in free-hand sections of stems through which 

 a colored solution has been lifted by transpiration, as in our former experi- 

 ments, small groups of the minute cells in the cambial ring which are colored. 

 These groups of cells which form strands running through the stem are pro- 

 cambium strands. The cells divide and increase just like the cambium cells, 

 and the older ones thrown off on either side change, those toward the center 

 of the stem to wood vessels and fibers, and those on the outer side to bast 

 cells and sieve tubes. 



107. Fibrovascular bundles in the Indian corn. We should now make 

 a thin transection of a portion of the center of the stem of Indian corn, in 

 order to compare the structure of the 



bundle with that of the plants which we 

 have just examined. In fig. 43 is repre- 

 sented a fibrovascular bundle of the stem 

 of the Indian corn. The large cells are 

 those of the spiral and reticulated and 

 annular vessels. This is the woody por- 

 tion of the bundle or xylem. Opposite 

 this is the bast portion or phloem, marked 

 by the lighter colored tissue at i. The 

 larger of these cells are the sieve tubes, 

 and intermingled with them are smaller 

 cells with thin walls. Surrounding the 

 entire bundle are small cells with thick 

 walls. These are elongated and the taper- 

 ing ends overlap. They are thus slender 

 and long and form fibers. In such a 



bundle all of the cambium has passed vessel ;"r annular vessel; /, air cavity 



r . formed by breaking apart of the cells ; /, 



over into permanent tissue and is said to ft bast, a form of sieve tissue ; /, thin- 

 , , , walled parenchyma. (Sachs.) 



108. Eise of water in the vessels. During the movement of the water or 

 nutrient solutions upward in the stem the vessels of the wood portion of the 

 bundle in certain plants are nearly or quite filled, if root pressure is active 

 and transpiration is not very rapid. If, however, on dry days transpiration 

 is in excess of root pressure, as often happens, the vessels are not filled with 

 the water, but are partly filled with certain gases because the air or other 



Fig. 43- 



Transection of fibrovascular bundle of 

 Indian corn. a, toward periphery of 

 stem ; g, large pitted vessels ; s, spiral 



