90 BIOLOGY FOR BEGINNERS 



If the blade of a leaf be cut across and studied with a micro- 

 scope, the following tissues may be observed. Mentioned in order 

 from the upper surface they are: 



1. The cuticle (sometimes lacking). 



2. The upper epidermis. 



3. The palisade cells. 



4. The spongy layer (traversed by veins). 



5. The air spaces. 



6. The lower epidermis (penetrated by stomates). 



The Upper Epidermis. This usually consists of a single layer 

 of cells often very irregular, as seen from above, but brick shaped 

 when viewed in cross section. There are few stomates in the 

 upper epidermis, since they would be exposed to dust and rain. 

 The function of the upper epidermis is to prevent loss of water. 

 To aid in this, it is sometimes covered by a waxy layer, called 

 the cuticle, as in ivy, cabbage, and other leaves that shed water 

 in drops. A second function of these epidermal cells may be to 

 act as lenses and concentrate the sunlight upon the inner parts 

 of the leaf. The fact that their upper and lower surfaces are 

 curved like a lens, leads to this supposition. 



The Palisade Layer. Next beneath the upper epidermis is the 

 palisade layer. It consists of long narrow cells, placed endwise, 

 at right angles to the surface of the leaf. Within these cells is 

 found the chlorophyll, which is the green coloring matter of all 

 plants. As you will learn later, it is very sensitive to light and 

 these long cells permit the chlorophyll grains to move to the upper 

 ends if the light be dim, or to retreat to the long side walls if the 

 light is too strong. 



The function of the palisade layer, then, is to regulate the ex- 

 posure of chlorophyll to light, and to carry on starch making. 



The Spongy Layer. Beneath the palisade layer is a spongy 

 layer which consists of thin-walled cells and air spaces, and is 

 penetrated in all directions by veins (duct bundles). The spongy 

 cells are roundish, irregular, and loosely packed, thin walled, and 

 full of protoplasm and chlorophyll. In them, as in the palisade 



