52 PLANT PHYSIOLOGY AND ECOLOGY 



system is a closed ring, encircling the pith. The continued growth 

 of this ring from year to year results in a stem consisting almost 

 wholly of woody tissues. 



66. Stem structure of an herbaceous dicotyledon. A section 

 of the stem of an herbaceous dicotyledon reveals an epidermal, 

 cortical, and fibrovascular region. The former is regularly a single 

 layer of cells, as in the root. Its cells are often differentiated into 

 epidermal cells, hairs, and guard-cells of stomata. The epidermal 

 cell has its outer wall rendered impervious, and often also thickened, 

 to prevent water loss. Hair-cells are epidermal cells elongated 

 after the manner of root-hairs. They are usually many-celled, 

 however, and their walls are cutinized, indicating that they are a 

 protection against water loss. Guard-cells are indirectly a result 

 of the cutinization of the epidermis. The latter renders necessary 

 the presence of openings for admitting gases, and the consequent 

 danger of water loss requires the development of guard-cells to 

 regulate the opening. All of these modifications of the epidermis 

 are really characteristic of the leaf, and they occur in the stem 

 only in so far as it assumes some of the same functions. 



The cortical region often consists of parenchyma alone. In 

 many plants, however, a secondary supportive tissue, the collen- 

 chyma, is developed next the epidermis, and, more rarely, one or 

 more rows of bundles of wood or stone fibers appear somewhere 

 in this region also. Milk-tubes are likewise found here. The 

 parenchyma is usually characterized by the presence of chloro- 

 plasts, though usually to a less degree than that of the leaf, and 

 of intercellular spaces for the passage of air. In most cases, it 

 does not show a differentiation of exoderm and endoderm, though 

 the latter often occurs as an incomplete sheath about the fibro- 

 vascular bundles. The fibrovascular system is usually in the 

 form of an interrupted circle of bundles strung like beads upon 

 the ring of cambium. In older stems, or in robust species, such 

 as the sunflower, the ring of bundles often becomes completely 

 closed. In a few cases, such as the cucumber, there is a secondary 

 outer ring. In many succulents, and in water plants, the fibro- 

 vascular bundles are greatly reduced, and their behavior excep- 

 tional. As in the root, each bundle has a phloem and a xylem 

 portion, though their relative position is now changed. They are 

 no longer radial, but the phloem has swung around so that it is 

 opposite and outside of the xylem. In most herbs, the phloem 



