NATURE OF PLANTS 11 



are joined to rather irregular and loosely arranged cells (the 

 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 / the end of a small vein is shown, having 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 extreme thinness of the wall at such places. 

 The term parenchyma is applied to all cells possessing the char- 

 acter 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 photosynthesis. 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 



