20 COMPOUND ORGANS OF PLANTS. 



tect those systems with their fluid contents against changes in 

 the state of the atmosphere, and to control the evaporation 

 from their cells within proper bounds. 



In the Lily and Balsam, which allow of ready evaporation, 

 the epidermis consists of a single layer of cells ; but in plants 

 which inhabit dry situations, it is so constructed as to retard 

 evaporation, and either consists of several layers of cells, as in 

 the Oleander, (Fig. 2,) or else is of considerable thickness, as 

 in the Aloe and Cactus. By this provision these plants are 

 enabled to retain their moisture for a greater length of time. 



Fig. 2. 



Magnified perpendicxilar section of the leaf of the Oleander, showing the thickness 

 of the epidermis, which is composed of three layers of cells, and the compact vertical 

 cells of the upper stratum of parenchyma. 



It must be evident that the exhalation of water from the 

 leaves is to a certain extent necessary, as it is the only means 

 by which the sap can be concentrated and rendered subservient 

 to the nutrition of the plant. Now so long as the roots can 

 absorb as much water as the leaves evaporate, the plant will 

 appear fresh and green, but the foliage droops (as is often seen 

 on a hot summer's day) when the supply at the roots fails, and 

 there is too much evaporation from the leaves. 



To remedy this defect, the epidermal surface of the leaves is 

 furnished with self-acting valves or openings, called stomata or 

 pores. These stomata are usually of an oval figure with a slit 

 in the middle, and are so situated as to open directly into the 



