SECONDARy THICKENING. NORMAL DICOVrLEDONS. 



53^ 



zones of secondary growth, to the non-equivalent members, and to individual dif- 

 ferences, there is far less to be said in the case of the bast than in that of the wood, 

 partly, no doubt, owing to the really greater simplicity of the phenomena, partly 

 because it is better to pass over a number of the known changes and to discuss them 

 in the next chapter, and lastly, in no small degree, because, for reasons already stated 

 above, there is still an absence of any very minute or extended investigations on the 

 subject. 



In woods, the ligneous elements and cambial cells of which increase succes- 

 sively for a time in size (p. 505), the same is generally the case in the bast, as would 

 be expected beforehand, on account of the enlargement of the cambial cells. This is 

 evident on observation in the case of numerous Coni- 

 ferae and Dicotyledonous woods, e. g. Tilia, Fagus, and 

 Nerium, also Vitis and Cobsea. It is obvious that 

 only those internal zones must here be taken into con- 

 sideration, which have not as yet undergone any sub- 

 sequent dilatation. No very accurate investigations 

 have been published on the degree and the persistency 

 of the general increase in size, nor on the possibly un- 

 equal participation of the particular forms of tissue in 

 these changes. 



Between stem and branches differences in the size 

 of the elements appear to exist similar to those in the 

 wood, but these also have not yet been more closely 

 investigated. In the roots of trees and shrubs, so far 

 as the investigations extend, the special structure of 

 the bast is very similar to that in the stem of the 

 same plant, e. g. Vitis, Sambucus, Tilia, and Punica, 

 Wherein the differences, which no doubt occur, con- 

 sist, cannot be stated at present. It has been repeatedly 

 mentioned above (pp. 516 and 524) that a different 

 relation exists between the foliage stems of herbaceous plants, and the roots which 

 belong to them, especially when the latter are fleshy. 



The thickness of the secondary zone of bast added in a definite space of time 

 is very variable, both in its relation to the simultaneous growth of the wood, and 

 according to absolute measure, whether in different species and individuals, or in 

 non-equivalent members of the same plant. In both relations the extreme cases are 

 represented on the one hand by the fleshy roots, consisting chiefly of bast (p. 516), 

 and on the other by the woody stems of trees and shrubs. 



Especially in trees with a persistent cortex, which is not thrown off by the 

 formation of bark (Sect. 177), as, for example, the Fir and Beech, the difference of 

 thickness between wood and bast is, as is well known, very considerable, and the 

 absolute thickness of the latter small. In the common Beech with smooth bark 

 (Fagus silvatica) the entire bast-aone in a stem 100 years old is, according to Hartig ', 

 scarcely more than i°i™ thick. Woody plants which periodically throw off their 



FIG. 215. — Punica Granatum (220) ; trans- 

 verse section through the inner part of tlie 

 bast of a branch six years old. c side towards 

 the cambium ; m, m two medullary rays ; 

 between the latter is a strand of bast ; s, s 

 sieve-tubes. It is uncertain which of the 

 other cavities belong to elements of the latter 

 kind. Between the empty cavities of the 

 elements of the soft bast are transverse zones 

 of crystal-containing sacs ; the clusters con- 

 tained in them are only indicated by shading. 



' Forstl. Culturpfl. p. 212. 



M m 2 



