14 THE STRUCTURE OF ECONOMIC PLANTS 



origin of a cell but its topographic relation to the axis and the 

 neighboring tissues which determines its future." Priestley and 

 Swingle C33), in their work on vegetative propagation, reach some- 

 what the same conclusion. 



Parenchyma — Potential Meristem. — The fundamental par- 

 enchyma occurring in various organs of the plant may be re- 

 garded as potentially meristematic in contrast to the active 

 meristems; and, as long as these cells retain their functional pro- 

 toplasts, they also have the capacity for further development and 

 differentiation which may or may not be expressed later. This 

 characteristic of parenchyma explains the origin of many of the 

 anomalous or adventitious structures that arise in the later develop- 

 ment of the plant body. Some of the parenchymatous cells may 

 achieve maturity without further differentiation except for an 

 increase in cell size and wall thickness. Such cells form the funda- 

 mental ground tissue which constitutes, at least in part, the pith, 

 cortex, and rays of the stem axis; the cortical region of the root 

 and the pith when present; and the mesophyll. In the leaf, the 

 parenchyma is often characterized by the presence of chloroplasts 

 and is referred to as chlorenchyma; but, in addition to the photo- 

 synthetic function, these cells store food for short or relatively 

 long periods. 



Where no potentialities are expressed, the parenchymatous cell 

 may maintain itself as such throughout the major portion of the 

 life of the plant; and, in some woody perennials, the cells of the 

 ray tissue remain alive for many years. In other cases, they grad- 

 ually become necrotic; or they may disintegrate early in ontogeny 

 so that lysigenous or schizogenous cavities occur. The hollow 

 medullary cavity frequently found in mature herbaceous stems such 

 as Triticum and Cucurbita, the large lacunae in the vascular bundles 

 of Zea, and those in the cortex of the stem of Pisum are formed 

 in this manner. 



The Pericycle. — The pericycle is the peripheral layer of the 

 stele, and may be regarded as a potential meristem. In the early 

 ontogeny of the axis, it commonly consists of a single layer of 

 parenchymatous cells lying immediately centrad to the endo- 

 dermis. The pericyclic cells have no morphological characteristics 

 which distinguish them from other parenchymatous cells; and 

 were it not for their special relation to the stele and the variety of 

 ultimate cell types that may be derived from them, they would not 



