. Elementary botany. Botany. h I. Fig. 42. 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
. Elementary botany. Botany. h I. Fig. 42. 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 will see that the wall which formerly separated the cells has nearly or quite disappeared. In other words the two cells have now an open communication at the ends. This is so for long distances in the stem, so that long columns of these large cells form tubes or vessels through which the 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 overlap. In this way they serve to strengthen the stem. 105. 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. 42.) The 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. The tubes thus formed are therefore called sieve tubes and they extend for long dist
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