278 PRACTICAL BOTANY 



in the order of their increasing complexity. This is a woody 

 stem composed of several kinds of stem tissues. Some of these 

 tissues are very heavy-walled and give rigidity to the stem. 

 The heavy-walled cells (sclerenchyma) may be around the 

 outside as well as within the stem (Fig. 230). Other tissues 

 are composed of large pithy cells with thin walls (parenchyma). 

 These cells are sometimes stored full of food in the form of 

 starch. Still other tissues consist chiefly of rounded, fiber-like 

 bundles which extend lengthwise throughout the stem. These 

 are the fibrovascular bundles, which means " fibrous bundles of 

 vessels." A careful inspection of one of these bundles discloses 

 two kinds of tissues composing it, one usually of large cells 

 with hard, heavy walls (the xylem), and the other with thin- 

 walled cells (the phloem). It is customary to speak of the 

 xylem bundle and the phloem bundle, in which case we should 

 have in the fern a compound bundle, the phloem being ar- 

 ranged concentrically about the xylem. About these is a layer 

 of cells known as the endodermis, meaning " inside epidermis." 

 The fibrovascular bundles are special conducting tissues. 

 Through these water and materials in solution in it are carried 

 to the leaf, and manufactured foods are carried back to stem 

 and roots. It is thought that the water is carried upward 

 chiefly through the xylem, and that manufactured food is 

 carried downward through the phloem. The fibrovascular 

 bundles extend throughout the roots and leaves. 



258. The fern plant : the leaf. By means of a transverse 

 section of the leafstalk it is seen that conducting bundles and 

 sclerenchyma are present as in the rhizome, though the arrange- 

 ment differs in some ways. The hard surface of the leafstalk 

 is due to the presence of sclerenchyma, and its strength par- 

 tially to sclerenchyma and partially to fibrovascular bundles. 

 The fibrovascular bundles extend by branches into the green 

 leaflets, where they are recognized as the veins of the leaf. 



By removing the leaf surface (epidermis) and examining 

 with a microscope its structure may be seen. It consists of a 

 single layer of cells whose irregular walls fit into one another 



