44 



PRINCIPLES OF PLANT CULTUBE 



more or less cubical or globular in outline. But germi- 

 nation scarcely commences before some of the cells begin 

 to increase greatly in length without a corresponding 

 increase in thickness. Cells of the former class, i.e., 

 those that retain their globular shape, are called paren- 

 ch^'ma, and those of the latter class prosenchyma (Fig. 

 20). Fig. 17 shows parench^Tna cells from the apple leaf. 

 The elongated cells form in groups or 

 bundles (^'ascular bundles, also called 

 fibro-vascular bundles) that extend 

 lengthwise through the stem and roots, 

 and since the individual cells overlap 

 and are in intimate contact, they form 

 threads often visible to the naked eye. 

 By the absorption of the ends of some 

 of the cells, tubes (ducts or vessels) of 

 very considerable length are formed. 

 In other cells of the vascular bundle, 

 the walls are much thickened and 

 strengthened by woody deposits. These 

 vascular bundles serve the double pur- 

 pose in the plant of strengthening and 

 conducting tissue. The fibers are espe- 

 cially for strength while the vessels 

 serve chiefly to conduct water and dissolved food mate- 

 rial or sap to the different parts. These groups or bun- 

 dles of fibers and ducts divide and subdivide in the leaves, 

 forming the so-called veins and veinlets. In the roots 

 they divide in a similar manner, extending lengthwise 

 through all the branches and branchlets. Fig. 20 shows 

 a longitudinal section of a vascular bundle of the rye 

 plant and Fig. 21 shows a cross-section of a vascular 

 bundle of the sunflower. 



Fig. 20. — Prosen- 

 chyma cells from 

 stem of rye. 

 Highly magni- 

 fied. 



