280 TISSUES 



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

 form, as in the epidermis of leaves (Fig. 137) ; and their 

 outer surface bears a layer of cuticle, a protective sub- 

 stance which is insoluble even in sulfuric acid. They do not 

 bear chloroplasts and often contain only cell-sap, with a 

 little protoplasm. Their walls are much thickened in some 

 cases, as in Figs. 447 and 471. Hairs and bristles are con- 

 sidered to be modified epidermal tissue. 



475. Collenchymatous Tissue. — Tissue composed of cells 

 thickened at the angles, not much elongated and not lapping 

 at the ends, is known as collenchyma. (Fig. 

 449.) It is strengthening tissue. Good 

 examples are found in such vines as 

 pumpkin, cucumber and gourd. The tis- 

 sue is slightly elastic and allows of some 

 stretching. Cut a few thin cross-sections 

 ,, of large stems of iewel-weed, and mount 



449. Collenchyma in wild ° ' 



jewel-weed or touch- in water. Study with high power. 



me-not (impatiens). ^ ^ g^ of g.^ Tissue# _ In the 



higher plants is a tissue known as soft bast or sieve tissue (this 

 also forms part of the bundle; 476). It is composed of two 

 types of cells which almost always accompany each other. 

 These are sieve tubes and companion cells. (Fig. 450.) Both 

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

 tubes are so called because of the sieve-like areas that 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. 450.) They serve to connect the cell- 

 cavities with each other, and through them the proto- 

 plasm strands extend, as shown in the figure. 



477. Prosenchymatous Tissue. — Several elongated and 

 strong tissues, that greatly strengthen the stems in which they 

 are found, are collectively known as prosenchyma. The cells 

 of these tissues become much thickened by the addition of 

 layers to the inner surface, and finally lose their protoplasm. 



