364 PLANT MORPHOLOGY 



stomata; the palisade tissue, generally comprising one or two layers of 

 green cells, vertically elongated, and lying beneath the upper epidermis; 

 the spongy tissue, a loose region of green cells and large intercellular 

 spaces; the veins, vascular bundles that traverse the spongy tissue. The 

 arrangement of the conducting tissues in the veins is collateral, the xylem 

 lying above the phloem. Cambial activity, if present, is weak. The 

 mesophyll, including the palisade and spongy tissues, is the photosyn- 

 thetic tissue of the leaf. 



Root Tip. The root tip comprises the rootcap, embryonic region, 

 region of elongation, and region of maturation. The rootcap is a protec- 

 tive sheath. The embryonic region includes the apical meristem, of very 

 limited extent, characterized by active cell division. In the region of 

 elongation the newly formed cells increase in length, while in the region of 

 maturation they become differentiated to form permanent tissues. In 

 many roots three or four distinct growing regions, or histogens, can be 

 recognized (Fig. 310). The outermost layer of cells, nearly continuous 

 around the embryonic region, is the dermatogen, which gives rise to the 

 epidermis. Inside the dermatogen is the periblem, consisting of several 

 layers that form the cortex. In the center of the root tip is the plerome, 

 which produces the stele. In the monocotyledons a calijptrogen forms the 

 rootcap and lies just behind it. 



The plerome arises from a group of initials situated at its very tip. 

 Just beyond lie another group of initials, often constituting a single layer. 

 In the dicotyledons these form the periblem, while a third layer, beyond 

 and in contact with it, gives rise to both the dermatogen and rootcap. 

 In the monocotyledons the middle group of initials produce both the 

 periblem and dermatogen, while the outermost layer of initials, the calyp- 

 trogen, independently forms the rootcap (Fig. 310). 



Root hairs, arising in the region of maturation, are slender tubular 

 extensions of the epidermal cells. They greatly increase the absorbing 

 surface of the root. New ones are formed as the root increases in length. 

 The older ones finally disappear. 



Mature Root. The structure of the mature root is rather uniform 

 throughout the angiosperms (Fig. 311). As in other vascular plants, the 

 root represents a primitive type of vascular organization, being typically 

 an exarch radial protostele. The stele is surrounded by an extensive 

 cortex whose innermost cells, the endodermis, have more or less thickened 

 walls. Lying immediately inside the endodermis is a layer of pa- 

 renchyma, or occasionally several layers, forming the pericycle. 



The process of lignification, progressing centripetally from the proto- 

 xylem strands, often does not reach to the center of the root, whose cells 

 then remain parenchymatous and form a pith. Such a condition is com- 

 mon in monocotyledons, while in dicotyledons a pith is typically absent. 



