The Inner Structure of the Plantlet. 



51 



68. The Vascular (vas'-cu-lar) Bundles. While the 

 plantlet remains within the seed-case, it consists largely 

 of cells more or less cubical or globular in outline. But 

 germination scarcely commences before some of the cells 

 begin to increase greatly in length without a correspond- 

 ing increase in thickness, f These 

 elongated cells form in groups or 

 bundles (vascular bundles) that extend 

 lengthwise through the stem and roots, 

 and since the individual cells overlap 

 and are in intimate contact, they form 

 threads or fibres. These fibres serve 

 the double purpose of giving strength 

 to the plant and conducting water, 

 with its dissolved food materials, to 

 the different parts. By the absorp- 

 tion of the ends of some of the cells, 

 tubes (ducts) of very considerable 

 length are formed. In other cells of 



FIG. 20. Prosenchyma 



cells from stem of rye. vascular bundles, the walls are much 

 'an d thickened and strengthened by woody 

 These groups or bundles of 



deposits. 



fibres 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. 21 shows a cross-section of a vascular bundle of 

 the sunflower. 



The threads in the stalk of Indian corn and the leaf- 



* Also called flbro-vascular bundles. 



t Cells of the former class are called Parenchyma (pa-ren'-chy-ma), and 

 those of the latter class prosenchyma (pro-sen'-chy-ma) (Fig. 20). Fig. 17 

 shows parenchyma cells from the apple leaf. 



