6o 



PLANT BIOLOGY 



the top ever develop ? Where do suckers come from ? 

 Where does the ear grow ? 



Cut a cross-section of the stalk between the nodes (Fig. 

 69). Does it have a distinct bark ? The interior consists 

 of soft "pith" and tough woody parts. The wood is found 

 in strands or fibers. Which is more abundant ? Do the 

 fibers have any definite arrangement ? Which strands are 

 largest? Smallest? The firm smooth rind{ which cannot 

 properly be called a bark) consists of small wood strands 

 packed closely together. Grass stems are hollow cylinders; 

 and the cornstalk, because of the lightness of its contents, 

 is also practically a cylinder. Stems of this kind are ad- 

 mirably adapted for providing a strong support to leaves 

 and fruit. This is in accordance with the well-known law 

 that a hollow cylinder is much stronger than a solid 

 cylinder of the same weight of material. 

 Cut a thin slice of the inner soft part and 

 hold it up to the light. Can you make out 

 a number of tiny compartments or cells ? 

 These cells consist of a tissue called paren- 

 chyma, the tissue from which when young all 

 the other tissues arise and differentiate (Paren- 

 chyma = parent + chyma, or tissue). The 

 numerous walls of these cells may serve to 

 brace the outer wall of the cylinder ; but their 

 chief function in the young stalk is to give 

 origin to other cells. When alive they are 

 filled with cell sap and protoplasm. 



Trace the woody strands through the nodes. 

 Do they ascend vertically ? Do they curve 

 toward the rind at certain places ? Compare 

 their course with the strands shown in Fig. 70. The ivoody 

 strands consist chiefly of tough fibrous cells that give rigidity 



FIG. 70. DIA- 

 GRAM TO SHOW 

 THE COURSE OF 

 FIBRO-VASCU- 

 LAR BCNDI.KS 

 IN MONOCOTY- 

 LEDONS. 



