254 TISSUES 



ducts and vessels ; the endodermis remains as endodermis; 

 periblem becomes the cortex of parenchyma; the derma- 

 togen becomes the epidermis of the root. 



411. EPIDERMAL TISSUE. This is a special modification 

 of parenchyma, comprising the thin layers on the exterior 

 of leaves and stems. The cells are often tabular or plate- 

 like in form, as in the epidermis of leaves (Fig. 115) ; 

 and their outer surface bears a layer of cuticle, a protec- 

 tive substance which is insoluble even in sulfuric acid. 

 They do not bear chlorophyll and often contain only cell- 

 sap, with a little protoplasm. Their walls are much thick- 

 ened in some cases, as in Figs. 394 and 414. Hairs and 

 bristles are considered to be modified epidermal tissue. 



412. COLLENCHYMATOUS TISSUE. Tis- 

 sue composed of cells thickened at the 

 angles, not much elongated and not 

 lapping at the ends, is known as collen- 

 chyma (Fig. 396). It is strengthening 

 tissue. Good examples are found in 

 396. couenchym'ainwiid such vines as pumpkin, cucumber and 



jewel-weed or touch-me- gourd. The tlSSUC is slightly elastic 



and allows of some stretching. Cut a 

 few thin cross -sections of large stems of jewel -weed, and 

 mount in water. Study with high power. 



413. SOFT BAST OR SIEVE TISSUE. In the higher plants 

 is a tissue known as soft bast or sieve tissue (this also 

 forms part of the bundle; 424). It is composed of two 

 types of cells which almost always accompany each other. 

 These are sieve tubes and companion cells (Fig. 397). 

 Both are elongated, thin -walled and blunt at the ends. 

 The sieve tubes are so called because of the sieve -like 

 areas which they bear in various parts. These areas, called 

 sieve plates, are commonly at the ends (as partitions) but 

 may be in the lateral walls. Fig. 397. They serve to 

 connect the cell - cavities with each other, and through 



