TEE TISSUES OF PLANTS. 23 



tissue is closely related to soft tissue, of which it is con- 

 sidered by some botanists to be a variation. 



40. Thick-angled tissue is found beneath the epidermis 

 of most flowering plants (and some ferns), usually as a 

 mass of considerable thickness, and is doubtless developed 

 from soft tissue for the purpose of giving support and 

 strength to the epidermis. 



Practical Studies. — {a) Examine a leaf-stalk of the squash or 

 pumpkin, and note the whitish bands, one or two millimetres wid6, 

 which extend from end to end just beneath the epidermis. These 

 are bands of thick-angled tissue. They may be readily torn out, 

 when the stalk will be found to have lost much of its streng-th. 



(J) Make a very thin cross-section of the preceding leaf stalk, and 

 note the appearance of the thick-angled tissue first under a low 

 power and then under a higher. The sections must be made exactly 

 at right angles to the axis of the bands of tissue in order to show 

 well. 



(c) Make a number of longitudinal sections of the same leaf-stalk, 

 in each case cutting through a band of the thick-angled tissue. Some 

 of these will show the thickened angles, although there is always 

 some difficulty in making them out in this section. 



(d) The stems of squash, pumpkin, pigweed, or lamb's-quarters 

 (Chenopodium), beet, and many other plants may be taken up next, 

 and their thick-angled tissue studied in cross and longitudinal sec- 

 tions. 



41. Stony Tissue {Sclerenchyma). — In many plants the 

 hard parts are composed of cells whose walls are thickened, 

 often to a very considerable extent (Pig. 14). The cells 

 are usually short, but in some cases they are greatly elon- 

 gated; they are sometimes regular in outline, but more 

 frequently they are extremely irregular. They do not 

 contain chlorophyll, but in some cases (e.g., in the pith of 

 apple-twigs) they contain starch. 



Practical Studies. — (a) Break the shell of a hickory-nut, and 

 after smoothing the broken surface cut off a very small thin slice ; 

 mount in water and a little potassic hydrate: the cell- walls are so 

 greatly thickened as to almost obliterate the cell-cavity. 



