192 -PART II. — VEGETABLE HISTOLOGY. 



The mesophloeum beneath this layer consists largely of paren- 

 chyma cells, the outer layers of which are rich in chlorophyll. 

 It is hence often called the green layer of the bark, for it is this 

 which in young shoots, before the formation of a periderm, com- 

 municates the green color to the surface, the cells of the epider- 

 mis being transparent. Some stems remain green for years 

 because of the delay to form a periderm, and the persistence of 

 the epidermis. Menispermum Canadense affords an example. 



Not infrequently there occurs at the junction of this layer 

 with the liber a ring or zone ; it may be more or less interrupted, 

 consisting partly of liber fibers and partly of stone cells. Some- 

 times where this zone crosses the medullary rays it dips inward 

 toward the centre of the stem, thus presenting a scalloped 

 appearance when viewed in cross-section. Clusters of stone 

 cells may also develop elsewhere in this layer, and it may 

 contain milk-tissues, secretion cells, secretion reservoirs, etc. 

 In stems where the layer persists, new cells are also formed 

 either along the continuations of the medullary rays or elsewhere, 

 so that it keeps pace with the general expansion of the tissues 

 of the stem. 



It is by no means always the case that the periderm originates 

 at or near the exterior. In many woody stems it originates inter- 

 nally, in the deeper layers of the mesophloeum, or even in the 

 innermost layer of cells where it joins the zone of liber fibers 

 just described, or if that is not present, where it joins the other 

 tissues of the liber. Thus, when the periderm is formed, not 

 only may the epidermis disappear but also the greater part or 

 even the whole of the mesophloeum, leaving only the liber 

 covered with a periderm. The Juniper, Currant, Honeysuckle, 

 Deutzia, Philadelphus and Barberry are examples of plants that 

 form an internal periderm. 



In the majority of woody stems when they become old, 

 whether the periderm is at first produced superficially or from 

 more deeply lying tissues, secondary formations of periderm 

 occur in succession interior to those first formed, and the layers 

 of dead tissues exterior to them, stretched by the growth of the 

 stem, become ruptured or fissured in various ways and peel off 

 from the surface. These secondary layers of periderm may be 

 formed in the mesophloeum or in the older portions of the liber ; 



