SECONDARY THICKENING. NORMAL DICOTYLEDONS. 



5^3 



In the bast of Dicotyledonous woody plants, the arrangement of the two kinds 

 of tissue, so far as can be decided from the existing data, is less regular than in the 

 Coniferous woods first mentioned, owing to the fact that the tangential series of the 

 one form of tissue are sometimes single, sometimes double or multiple, and are 

 interrupted by interpolated elements of the other form ; while the average width 

 of the adjoining tissue-elements of the two kinds not uncommonly shows considerable 

 differences, which are usually in favour of 

 the sieve-tubes, e. g. Tilia and Vitis ; more 

 rarely in favour of the parenchymatous 

 cells. The narrower parenchymatous cells 

 which accompany the sieve-tubes here show 

 the same arrangement and the same 

 characteristics as the camhiform cells de- 

 scribed at p. 324, in the case of the 

 primar}' vascular bundles ; they should 

 therefore be designated by the same 

 name. They appear in transverse section 

 as narrow, three or four-cornered meshes 

 contiguous with the tubes, sometimes on 

 one side of them, sometimes on more 

 than one, but never (.?) on all sides, each 

 side, however, never having more than one 

 (Fig. 212). 



Traced longitudinally (Fig. 213) they 

 form, in most cases at any rate, series, 

 each member of which is many times 

 shorter than the adjoining member of the 

 sieve-tube, and is derived from the trans- 

 verse division of the tissue mother-cell. In 

 Tilia I rarely found them equal in length 

 to the members of sieve-tubes. The fre- 

 quency of the narrow cambiform cells 

 seems to be very unequal in the different 

 particular cases ; for example, I find few 

 of them in Pyrus and Spiraea ulmifolia, 

 while they are numerous in Tilia. More 

 accurate details are to be expected from 

 further investigations. 



In spite of the irregularities described, 

 the radial and tangential seriation of the elements continues to be maintained in 

 many cases in its principal outlines, and each tangential row contains for the most 

 part either sieve-tubes or parenchyma, as is especially shown by longitudinal 

 sections which follow its course. The most various plants, e.g. besides those 

 mentioned, Populus, Salix, Punica, Ficus, Sambucus, Fagus (Mohl), ^sculus 

 and Ribes, show the greatest agreement in this respect, although great differences 

 occur between them in the average size of the elements and the special form of 



Fig. 213. — Vitis vinifera; bast of a branch several years old, 

 I cm. in thickness, in summer (beginning of July). Tangential 

 section (145). s,s sieve-tubes, the inclined scalariform terminal 

 surfaces, and one which is horizontal, being cut through longi- 

 tudinally, with the exception of one at the upper edge, which is 

 seen obliquely in superficial view, nt, ni medullar}' rays ; on the 

 boundary between these and the strand of sieve-tubes are sacs 

 containing crystals. 



