532. 



THE STRUCTURE OF ECONOMIC PLANTS 



The subterranean portion of the stem is similar to the aerial 

 in general plan, but differs from it in certain characteristics. 

 There is a decrease or complete absence of collenchyma in the 

 cortex; and immediately beneath the epidermal layer, the cells 

 tend to become more or less thickened at points where the epidermis 

 is ruptured. The epidermal cells are heavily suberized and Cas- 

 parian strips in the endodermis are clearly defined. 



Anatomy of the Rhizome. — The rhizome is stem-like, and its 

 ontogeny resembles that of the aerial shoot. The meristem has 

 the three zones, dermatogen, procambium, and fundamental meri- 

 stem, that occur in the aerial stem. The dermatogen forms a 

 single-layered epidermis consisting of square or hexagonal cells 

 that are at first approximately isodiametric, but later become 

 elongated in the axial direction. The outer wall is somewhat 

 arched and is thicker than the inner one with a thin cuticle. Some 

 of the epidermal cells are specialized to form guard cells and hairs, 

 and both stomata and hairs are produced in limited numbers 

 but are ephemeral because of the early development of the 

 periderm. 



The cells of the meristem enlarge without differentiation to 

 form the cortical zone external to the procambial ring and the 

 medullary zone within it. The cortex consists of a band of paren- 

 chymatous cells eight or nine layers in width which are rounded 

 and isodiametric or slightly elongated. Near the periphery, the 

 cells tend to be collenchymatous, but to a much less degree than in 

 the aerial stem. Small intercellular spaces are formed between the 

 cells in the middle cortical region. The cells adjacent to the 

 endodermis are smaller and approximately the size of the endo- 

 dermal cells in tangential dimension. The first visible storage 

 product, starch, can be detected in the cells adjacent to the pro- 

 cambial ring; and, later, larger deposits occur in the intermediate 

 cortical cells. The peripheral cells are usually devoid of starch, 

 but protein granules occur in increasing abundance as the matura- 

 tion of the tuber proceeds. 



The pith of the young rhizome is small as compared with the 

 cortex, and the cells comprising it are axially elongated. Both 

 pith and cortical cells are profusely pitted with groups of simple 

 pits. The development of the vascular tissue from the procambium 

 is similar to that of the aerial stem, but there is proportionately 

 more phloem produced than xylem. The outer phloem groups 



