TISSUE SYSTEMS 345 



bundle surrounded by a perieycle and endodermis, we have the 

 condition called astely or schizostely. 



This differs from polystely in that each stele in the latter case 

 is composed of two or more vascular bundles. 



The formation of the several steles in polystelic stems is due 

 to the differentiation of the cells of the plerome varying at 

 different times during the growth of the axis or stem. The 

 endodermis of the steles is not in this case, and in that of astely, 

 derived entirely from the periblem, but part of it springs from 

 the plerome. 



The Perieycle. — As we have seen, this portion of the stele 

 is to be regarded as ground tissue. It presents certain charac- 

 teristic features in different portions of the axis. Generally, its 

 cells are thin-waUed and parenchymatous, and form a continuous 

 layer over the stele. In the stem it is usually several layers of 

 cells thick, and may contain a variety of tissues, including bands 

 or strands of coUenchyma, sclerenchyma, or glandular tissue. 

 In the root it is usually only one cell thick, and the cells are 

 all parenchymatous. An exception is found in nearly all 

 Gymnosperms, where it is many -layered. There is, however, a 

 good deal of variety in its construction in many roots. It is not 

 always continuous, being absent behind the woody bundles of 

 many grasses and sedges and some other Monocotyledons, in 

 which the protoxylem cells abut directly on the endodermis. 

 In other cases its continuity is interrupted behind the bundles 

 of the bast, particularly in several aquatic Monocotyledons. 



Though generally of uniform thickness in roots, this is not 

 without exception. It may be several cells thick opposite to the 

 bast bundles, as in the aerial roots of some Orchids, or behind 

 the wood bundles, as in some Leguminoscp. Where it is many- 

 layered the cells may become sclerotised. 



The perieycle is often the seat of secondary formations, due 

 to certain of its cells becoming merismatic. Phellogens espe- 

 cially arise there, leading to the development of layers of cork. 

 This is particularly the case in roots, where such phellogen 

 extends completely round the stele. Another meristem giving 

 rise to vascular elements is also developed in the perieycle of 

 the root ; but this is discontinuous, occurring only outside the 

 bundles of the wood. Its separate parts join a number of strands 

 of a similar meristem arising more deeply in the stele, forming 

 thus a sinuous ring of generating tissue, or ca/rnhmin. In all 

 plants above the Vascular Cryptogams the perieycle gives rise 

 to the lateral rootlets. 



