NATURE OF PLANTS 89 



localized in a layer of cells situated on the inside of the lenticel 

 (Fig. 51). This layer of cells is a part of the cork cambium, 

 but strangely enough only loose cells are added to the lenticel 

 during each year's growth and consequently a passage way is 

 kept open to the living cells within the stem. The lenticels 

 appear as minute points upon the surface of young stems but 

 upon old trunks they often become greatly elongated, forming 

 the characteristic bands on the bark of the birch and cherry, etc. 

 (Fig. 52, B). In bottle cork, derived from the cork oak of the 

 Mediterranean, the lenticels appear as minute lines often errone- 

 ously referred to as worm holes. Usually as the cork layer 

 increases in thickness the cortical cells become less active, lose 

 their chlorophyll and the cork cambium closes the lenticels by 

 forming compact cork cells instead of the loose cells of the 

 lenticels. This same closure of the lenticels is frequently seen 

 in the fall but the renewal of growth in the spring results in the 

 formation of new loosely related cells which rupture the over- 

 lying cork layer and so open the lenticels. So it seems probable 

 that the lenticels arise owing to the active respiration and 

 transpiration of the cortical cells and, when these functions di- 

 minish, that the lenticels become closed and finally fail to develop 

 further. 



39. The Cambium Cylinder. Let us now consider the changes 

 that are effected in the vascular bundles through the activity of 

 the cambium. The increase in the diameter of the stems of 

 many annual plants and especially of our trees and shrubs is 

 largely brought about by the formation of new cells derived 

 from the cambium. While the vascular bundles are very small 

 it will be noticed that the parenchyma cells separating the vascu- 

 lar bundles begin to divide so as to form a line of cells connecting 

 the cambium of each bundle (Fig. 53). In some cases these 

 divisions are at first somewhat irregular but soon the growth 

 results in the formation of cells with parallel walls and in this 

 way a line of regular cells is formed which in cross section appear 

 as a band or ring of cells but in longitudinal extent they constitute 

 a cylinder. These cells are therefore termed the cambium cylin- 

 der and they continue to divide as already noted in the cambium 

 7 



