60 



COMPENDIUM OF GENERAL BOTANY. 



B = cork-cambium). Usually it begins in a more deeply located 

 layer of the parenchyma (Fig. 35). In roots the seat of cork- 

 formation is, as a rule, found in the 2>^ri- 

 camhium. Concerning this pericambiura, 

 we will at this point state only that it is 

 a tissue one or more layers in thickness, 

 lying within the primary root-parenchyma 

 outside of the centrally located vascular 

 bundle. 



Cork is a complex structure, com- 

 posed of different elements, but its origin 

 can be easily determined. As a rule, it is 

 developed according to a twofold plan — 

 either as ring-co7']t, or as scaly cork. 

 From the nature of things tissues which 

 are separated from the sap-bearing tissue 

 Fio. 36.-TWO stages of cork- «! ^^^^ interior by a corky layer are subject 

 formation in the stem of Scu- to desiccation. It is also a rule that one and 

 tellaria splendens. i i • i 



(After Haberiandt.) the samc cork-cambium does not possess 



an unlimited power of growth, as is the 

 case in the cambium-ring of our trees. The cork-cambium discon- 

 tinues its cell-forming activity, while a new zone of cork-cambium 

 appears more in the interior ; this new layer bears the same relation 

 to others, etc. Either these successive cork-layers have the form 

 of continuous cylinders,— in which case they appear as rings in 

 cross-section, and the bark peels off in cylindrical pieces, — or the 

 successively formed cork-cambiums (and their products) have the 

 form of watch-crystals or similar curved surfaces whose convexities 

 are directed inward, appearing as partial circles in cross-section, and 

 in some cases {Platanus, for example) forming scales which peel off 

 very perfectly, leaving the stem quite smooth; in other cases tlie 

 scales remain attached in large numbers for some time, the bark 

 becomes very rough with deep crevices, and the scales are thrown 

 off at irregular intervals. Hence "bark" at first contains the 

 elements of the primary parenchyma between its cork-lamellae, later 

 also those of the secondary parenchyma, still later only those of the 

 secondary parenchyma. 



Besides the above-mentioned conditions in the case of birch-bark 

 {Betuld)., thin-walled and thick-walled unsuherized ae\\%, which are 

 intercalated between the suberized cells, are sometimes formed in 



