52 



PHYSIOLOGY. 



places than in others, and this thickening takes place so regularly in some 

 instances as to form regular spiral thickenings. Others have the thickenings 



Fig. 59. 

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

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

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



in the form of the rounds of a ladder, while still others have pitted walls or the 



thickenings are in the form of rings. 



103. Vessels or ducts. — One way in which the cells in side view differ 

 greatly from an end view, in a cross section in the bundle, is that they are 

 much longer in the direction of the axis of the stem. The cells have become 

 elongated greatly. If we search for the place where two of these large cells 

 with spiral, or ladder-like, markings meet end to end, we see that the 

 wall which formerly separated tlie cells lias nearly or quite disappeared. In 

 other words the two cells have now an open communication at tlie ends. 

 This is so for long distances in the stem, so tliat long columns of these large 

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

 plants. 



104. In the bast portion of the bundle we detect the cells of the bast fibers 

 by their thick walls. They are very much elongated and the ends taper out to 

 thin points so that they overla p. In this way they serve to strengthen tiie stem- 



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

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

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

 in sucli 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. 59.) Tlie granular appearance which these plates pre- 

 sent is caused by minute perforations through the wall so that there is a com- 

 munication between the cells. Tlie tubes thus formed are therefore called 

 sieve tubes and they extend for long distances through the tube so that there 



