NATURE OF PLANTS n 



spongy mesophyll, Fig. 5, sp). Numerous air spaces extend in 

 all directions through this spongy mass of cells reaching up to 

 and between the palisade cells and down to the stomata (Fig. 

 5, i). Bundles of small cells also appear here and there between 

 the palisade and spongy mesophyll (Fig. 5, v, I). These struc- 

 tures are the so-called veins that appear as fine lines in the 

 majority of leaves (Fig. 3). These veins or vascular bundles 

 branch again and again and so extend to all parts of the leaf. 

 In Fig. 5, v one of the veins has been cut square across while 

 at / a very small one has been cut in half lengthwise. The walls 

 of the mesophyll cells are smooth, thin, and elastic and composed 

 of a substance called cellulose. Thin places or pores are often 

 formed in the walls which appear as openings owing to the ex- 

 treme thinness of the wall at such places. The term parenchyma 

 is applied to all cells possessing the character of walls noted in 

 the mesophyll cells. In the vascular bundle, however, a portion 

 of the cells have thicker walls which are more rigid, hard and 

 woody. Such cells are often referred to as prosenchyma. 



What is the significance of the peculiar and varied arrange- 

 ments of the cells in the leaf ? We naturally anticipate that these 

 different structures are adapted to performance of special duties. 

 Attention will first be directed to the work performed by the 

 green portion of the leaf, the mesophyll. Similar cells containing 

 chloroplasts are of common occurrence in various parts of the 

 plant, being often found in stems and unripe fruits. In the leaf 

 such a tissue is termed mesophyll but a more general term, Chlor- 

 enchyma, includes all chlorophyll-bearing tissue wherever found. 



6. The First Function of the Leaf, Photosynthesis. — Chlor- 

 enchyma is the most important tissue in the plant since it has for 

 its special function or work the construction of carbohydrates, 

 such as sugars and starches, which are among the most impor- 

 tant foods of the plant. The construction of these carbohydrates 

 by the chlorenchyma is called photo-synthesis. This work easily 

 ranks as the leading wonder of the world because its processes 

 are so complicated and of such magnitude and importance in the 

 economy of this world of ours. It will be seen that this tissue 

 not only keeps the air in a wholesome condition for breathing 



