^20 SECONDARY CHANGES, 



of the former, it is sharply marked off fi-om the non-equivalent tissues of the external 

 cortex, more especially in those cases most frequent in stems, where the original groups 

 of phloem are supported or enclosed on the outside by sclerenchyma. Nageli' has 

 called this external limiting zone of the bast-layer the cortical sheath, a term correr 

 sponding to that of medullary sheath, used for the internal boundary of the wood,- 



The original structure of this limiting zone is evident from the descriptions given 

 in preceding paragraphs ; the structure of the bast as a whole depends partly upon 

 the former, partly upon that of the secondary increment of growth to be described 

 here. A further modification, however, results from the fact that the structure of th^ 

 bast must undergo a constant change, so long as the volume of the body enclosed by 

 it is increased by means of the activity of the cambium ; for each zone thus under- 

 goes, after its first formation, a constantly increasing extension in the direction of 

 its surface, by which it must be in some way or other affected. So long as growth in 

 thickness goes on, a constant transition must of necessity take place between the 

 original conditions and those which have been modified by the peripheral extension, 

 and in fact the successive stages of change must always come under observation in 

 every mass of bast which is regarded as a connected whole. This fact must always 

 be borne in mind. For purposes of description, however, it is necessary to separate 

 the initial structure from the changes due to peripheral extension. The former will 

 be immediately considered here, the latter in the next chapter. 



The differentiation of the bast (Sect. 135) is similar to that of the wood, in so 

 far as it consists of principal and partial strands of various degrees, which are 

 separated from one another, or divided up by the large and small medullary rays 

 (shortly termed hast-rays). The equivalent rays and strands of wood and bast 

 correspond, and fit on to one another in the cambial zone. The original form and 

 size of the medullary rays, and the consequent course of the strands, are the same 

 as in the adjoining wood. 



Sect. 161. Among 'Has. forms of tissue, sieve-tubes and parenchyma are charac- 

 teristic, without exception, of the secondary bast of Dicotyledons and Gymnosperms 

 with normal growth. They are, at least very frequently, accompanied by sacs con- 

 laining crystals (comp. p. 141); further, by sclerenchymatous elements, and especially 

 'by elongated fibrous cells, the bast-fibres ; not uncommonly also by short sclerenchyma, 

 sione-sclerenchyma (stone-cells) ; while, finally, laticiferous tubes and secretory reservoirs 

 characterise tlie bast of certain species or families. As has already been partly 

 shown by the descriptions of the forms of tissue given in former chapters, especially 

 Chap, v., all these elements in the bast, with the obvious exception of the scleren- 

 'chyma, possess delicate, non-lignified, soft walls. Nageli has accordingly introduced 

 .the collective term soft bast for all those portions of the bast which are not 

 ^sclerenchymatous. In most cases the elements of the soft bast are originally 

 Jiarrow, and continue to resemble the cells of the cambium, from which they are 

 derived; they are often difficult to distinguish from the cambium, and from one 

 another, especially as seen in transverse section. For this reason, but to a much 

 greater extent owing to the fact that the softness of the tissues makes it somewhat 

 ■difficult to obtain good preparations, the structure of the bast, and its distinction frotji 



' Dickenwachsthum, &c., d. Sapiiidaceeii, p. 13. 



