66 



BOTANY. 



cells form tubes or vessels through which the water rises in the stems of 

 plants. 



120. Bast fibres. — In the bast portion of the bundle we detect the cells of 

 the bast fibres by their thick walls. They are very much elongated and the 



.*"«• S3- 

 Longitudinal section of vascular bundle of sunflower stem; spiral, scalariform and pitted 

 vessels at left; next are wood libers with oblique cross walls; in middle are cambium cells 

 with straight cross walls, next two sieve tubes, then phloem or bast cells. 



ends taper out to thin points so that they overlap. In this way they serve to 

 strengthen the stem. 



121. Sieve tubes. — Lying near the bast cells, usually toward the cambium, 

 are elongated cells standing end to end, with delicate markings on their cross- 

 walls which appear like finely punctured plates or sieves. The protoplasm 

 in such cells is usually quite distinct, and sometimes contracted away from 

 the side walls, but attached to the cross-walls, and this aids in the detection 

 of the sieve tubes (fig. 53). The granular appearance which these plates 

 present is caused by minute perforations through the wall so that there 

 is a communication between the cells. The tubes thus formed are there- 

 fore called sieve tubes, and they extend for long distances through the 

 bundle so that there is communication throughout the entire length of the 

 stem. (The function of the sieve tubes is supposed to be that for the down- 

 ward transportation of substances elaborated in the leaves. ) 



122. Bundle in the sunflower stem. — In like manner a section of the stem 

 of the sunflower shows similar bundles, but the number is greater than eight. 

 In the garden balsam the number is from four to six in an ordinary stem 

 3-4*k/» diameter. Here we can see quite well the origin of the vascular 

 bundle. Between the larger bundles especially in free-hand sections of stems 



