44 VEGETABLE PHYSIOLOGY. [LESSON 15. 



261. The globular apex of glandular hairs sometimes forms a 

 viscid secretion, as in the Chinese Sundew (Drosera rotundifolia). 

 The hairs of this plant, by means of its peculiar secretion, are 

 enabled to detain insects which chance to alight on them (Fig. 78). 



LESSON XV. 



CUTICLE. 



262. This tissue in plants corresponds remarkably with the kin- 

 dred tissue in animals, especially in man. In both it is non-vascular, 

 transparent, more or less thick, and abundantly supplied with hair, 

 as we have just seen. 



263. In plants, it is usually formed by a layer or layers of com- 

 pressed cells, which assume a flattened shape, and have their walls 

 bounded by straight or by flexuous lines. Every leaf presents a cu- 

 ticle on the upper and on the under surface, each composed entirely 

 of cells, but in many plants dissimilar in the two cuticles. 



264. The cuticle is sometimes thin and soft, at other times dense 

 and hard. In the former case it may easily be detached from the 

 subjacent cells ; in the latter, the cells become thickened by deposits, 

 and sometimes the layers are so produced as to leave uncovered 

 spots, which communicate with the interior of the cell by canals pass- 

 ing through the thickened layers. 



265. In terrestrial plants the breathing mouths (stomata) are 

 placed, generally, wholly on the under cuticle, but some plants have 

 in addition a few scattered organs of this description on the upper 

 cuticle. The object of placing these important organs on the under 

 surface of the leaf, is to protect them from the rain, which would in- 

 terfere with the due performance of their function if it fell upon the 

 surface to which they were attached. 



266. Stomata open or close, according to the state of moisture or 

 dryness in the atmosphere. By examining under the microscope, 

 thin strips of cuticle in a moist and dry state, it will be seen that in 

 the former case the lips are distended ; they assume a crescentic or 

 arched form, and leave a marked opening between them ; while in 

 the latter, they approach each other, and close the orifice. 



267. The number of stomata varies from a, few hundreds to many 

 thousands on a surface of one inch square. 



