PART OF MOVEMENT OF LIQUIDS IN PLANTS. 65 



116. Bast portion of the bundle. — If our section is through a part of the 

 stem which is not too young, the tissues of the outer part of the bundle will 

 show either one or several groups of cells which have white and shiny walls, 

 that are thickened as much or more than those of the wood vessels. These 

 cells are bast cells, and for this reason this part of the bundle is the bast 

 portion, or the phloem. Intermingled with these, cells may often be found 

 which have thin walls, unless the bundle is very old. Nearer the centre of 

 the bundle and stil^within the bast portion are cells with thin walls, angular 

 and irregularly arranged. This is the softer portion of the bast, and some 

 of these cells are what are called sieve tubes, which can be better seen and 

 studied in a longitudinal section of the stem. 



117. Cambium region of the bundle. — Extending across the centre of 

 the bundle are several rows of small cells, the smallest of the bundle, and we 

 can see that they are more regularly arranged, usually in quite regular 

 rows, like bricks piled upon one another. These cells have thinner walls 

 than any others of the bundle, and they usually take a deeper stain when 

 treated with a solution of some of the dyes. This is because they are younger, 

 and are therefore richer in protoplasmic contents. This zone of young cells 

 across the bundle is the cambium. Its cells grow and divide, and thus in- 

 crease the size of the bundle. By this increase in the number of the cells of 

 the cambium layer, the outermost cells on either side are continually passing 

 over into the phloem, on the one hand, and into the wood portion of the 

 bundle, on the other hand. 



118. Longitudinal section of the bundle. — If we make thin longisections 

 of the vascular bundle of the castor-oil seedling (or other dicotyledon) so 

 that we have thin ones running through a bundle radially, as shown in fig. 

 53, we can see the structure of these parts of the bundle in side view. We 

 see here that the form of the cells is very different from what is presented in 

 a cross-section of the same. The walls of the various ducts have peculiar 

 markings on them. These markings are caused by the walls being thicker 

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

 some instances as to form regular spiral thickenings. Others have the thick- 

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

 walls or the thickenings are in the form of rings. 



119. 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 shall 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 



