THE BARK. 



71 



can come to want successively its exo- 

 phloeum, mesophloeum, and even the 

 outer part of its endophloeum. The study 

 of barks includes a close examination of 

 their cellular elements, as a preparation 

 for which histological work is absolutely 

 necessary. Examination of its gross 

 characters involves, as the more impor- 

 tant features, its extreme and average 

 thickness, its manifest layers, as seen 



with a lens on transverse or radial sec- 

 tion, their relative thickness, color, mark- 

 ings, consistency as shown by fracture, 

 their separability from one another, that 

 is, into laminae, together with the sur- 

 face characters of the latter, the external 

 color and level markings, the presence 

 and nature of parasites, and the color and 

 inequalities of the inner surface. 



The laminae do not depend entirely 

 upon different tissue composition. The 

 same tissue, produced at different times, 

 may present differences sufficient to re- 

 sult in different degrees of cohesion, as 

 well as markedly different color, at dif- 

 ferent depths, so that separation may 

 readily occur, or they may readily be dis- 

 tinguished in section. Groui)s or radial 

 or tangential rows of tissue elements, dif- 

 fering from those adjoining, frequently 

 produce gross markings on the section 

 surface. The fracture of barks or of 

 their individual layers is denominated in 

 general as being brittle or tough. Various 

 modifications are soft, earthy, granular, 

 horny, waxy, fibrous, splintery or flex- 

 ible. A bark may be flexible in one di- 

 rection and not in another. 



The outer surface is described in gen- 

 eral as being harsh, rough, downy, 

 smooth or shiny, and its lustre may be 

 waxy, vitreous, and so on. Some of the 

 elements causing roughness may require 



microscopical examination for their dem- 

 onstration, while others are otherwise 

 manifest. Care must be taken to distin- 

 guish between ridging and furrowing of 

 other kinds. One kind is caused by a 

 longitudinal wrinkling in drying, as in 

 young Calisaya (Fig. 359). Another is 

 owing to transverse (as in old Calisaya) 

 or longitudinal (in the same) fissuring 



(Fig. 3G0). Another is caused by the 

 elevation of corky ridges, or rows of 



corky warts, which may or may not be- 

 come confluent in variable degri^e (as in 

 Succirubra, Fig. 3G1). Fissures may 

 characteristically open in the crest of a 

 ridge or in the otherwise unchanged sur- 

 face. Most color variegations are due to 

 lichens or other parasites, and those due 

 to lenticels are also very common. 



A single color or shade of color of the 

 inner surface is rarely characteristic, as 

 it changes very greatly with age in keep- 

 ing; but a carefully arranged series of 

 them may be made diagnostic in many 

 cases. The important characteristics of 

 the inner surface depend upon the pro- 

 jecting bast bundles and contracting 

 medullary rays. Very rarely indeed is 

 the surface so free from these inequali- 

 ties that it can be properly described as 

 smooth. The slightest manifestation of 

 the bundles gives the Striate condition. 

 The striae must be examined as to 

 length, straightness, direction as con- 

 trasted with the axis of the bark, appar- 

 ent interconnection at the end, width, 

 elevation and sharpness, with the com- 

 plementary characters of the intervening 

 furrows or pits. Some barks show a 

 tendency to separate into laminae which 

 run obliquely out upon the inner face, 

 appearing there as partially separated 

 tongue-shaped splinters. 



