THE ANGIOSPERMAE : STEMS 



873 



whether of the paHsade form or not, there will usually be some water storage 

 tissue closely adjacent to it in the cortex. The cells of such a tissue are large 

 and thin-walled, and they not infrequently contain mucilage. As it is nearly 

 always in xerophytes that chlorenchyma occurs in the cortex the advantage 

 of a water reservoir close at hand is obvious. Indeed, in the Cacti, where 

 the stele is often relatively slender, the greater part of the stem tissue is water- 

 storing cortex, onlv a narrow outer zone being chlorenchymatous. Internal 

 glands and sclereid cells are frequently present in the cortex, the latter being 

 sometimes complexly branched and forming an important part of the skeletal 



A B 



Fig. 859. — Aristolochia siplw. Breakage of the zone of pericyclic fibres due to 

 radial growth of the stem and its repair by the ingrowth of parenchyma cells 

 from the cortex, which become lignified sclereids. {After Haberlandt.) 



system of the plants {e.g., Magnoliaceae) in which they occur. INlore often, 

 however, they are isolated and scattered and not of significant mechanical 

 value. 



After elongation of the young stem is over, a ring of sclerenchymatous 

 fibres may form in the cortex, as, for example, in Oak, Birch or Ash, 

 supplementing the collenchyma as a strengthening tissue. Growth in girth 

 soon and repeatedly ruptures this ring, but the gaps are made good by the 

 ingrowth of parenchyma cells into the radial spaces due to the ruptures, and 

 their transformation into sclereids (Fig. 859). Thus a composite mechanical 

 ring is formed of arcs of fibre cells separated by arcs of sclereids. 



One of the most striking departures from normal cortex formation is to 

 be seen in aquatics, where the cortex (Fig. 860), or in the case of Mono- 

 cotyledons the whole ground tissue (Fig. 861), is divided into large air spaces, 



