PLANT STRUCTURE 139 



serve as protective coverings for the delicate 

 parts beneath. As growth in thickness takes 

 place, the older cells become functionless and fall 

 away. This is especially true of the bark of 

 trees. 



(g) Heart-wood cells. Heart-wood cells are those 

 near the center of the stem, and have become 

 functionless so far as food conduction and stor- 

 age are concerned. Their purpose now is to 

 strengthen the stem and thus support the foliage, 

 so that all parts may be brought into. full sun- 

 light and better atmospheric conditions. 



(h) Leaf cells. In leaf cells, extremely active pro- 

 cesses of plant growth take place. Because of 

 their activity and the need of fresh and thin 

 tissue in which to manufacture plant food, the 

 duration of the life of leaves is short. Upon 

 the healthfulness and vigor of the leaves of the 

 plants, depends in a large measure the success 

 dr failure of the plant grower. 



(i) Epidermal cells. Epidermal cells are transparent, 

 mostly brick-shaped cells on the upper and the 

 lower surface of the leaves. The outer row 

 usually has thicker cell walls, which serve for 

 protection. It may also be variously elongated 

 into hair-like processes which serve the same 

 purpose. These epidermal cells are transparent 

 so that the sunlight may penetrate easily to the 

 palisade cells and spongy parenchyma beneath. 



(j) Guard cells. In the epidermal cells are variously 

 modified cells filled with protoplasm which is 

 very readily acted upon by sunlight and atmos- 

 pheric conditions. Under certain circumstances 



