SECONDARY THICKENING. NORMAL DICOTYLEDONS. 467 



cells is elongated in the same direction as they are ; in - the medullary rays of many 

 woody plants (Vitis, Begonia) they contain chlorophyll, and in winter small starch-grains 

 (Vitis, Aristolochia Sipho, &c.). Their cellulose walls are thin and delicate at the 

 time of active growth, yet even here the difference between the radial and tangential 

 surfaces, to be mentioned immediately, frequently and perhaps always appears, or is 

 at least indicated. At the commencement of the winter's rest, the tangential walls of 

 the elongated cells remain smooth and relatively thin ; the radial walls, on the other 

 hand, become considerably thickened, the highly refractive thickening mass being 

 interrupted by a single longitudinal row of roundish pits. In the medullary rays a 

 similar thickening takes place, and on their limiting surfaces towards the elongated 

 cells there is a formation of pits corresponding to that on the latter (Figs. 198, 199- 

 20 r). On the recommencement of the period of growth, the thickening mass appears 

 to be again dissolved, at least in part. 



These peculiarities of structure are shared by the cells of the young wood and 

 young bast, as well as by the cambium. These cells may also enter with the latter into 

 the condition of winter's rest. If, during the latter period, the zone of young secondary 

 growth between the mature wood and bast be investigated, concentrically and radially 

 ■ arranged layers of cells are found, with the radial walls thickened as described ; on 

 the one hand, they are sharply limited towards the youngest mature wood, which is 

 distinguished by its thick lignified walls ; on the other hand, towards the bast, though 

 here the limitation is less sharp, at all the points where the mature elements have 

 slightly thickened and non-lignified walls. The number of the concentric cambium- 

 like layers varies, frequently even in immediately contiguous radial rows; a fact which 

 finds its explanation in the want of uniformity in the course of the cell-divisions in 

 the latter, as mentioned above. In the simplest case only the single initial layer lies 

 between the mature elements of the wood and bast : in its perfect form I have ob- 

 served this only in the case of Juniperus communis (comp. below, Fig. 207). Usually 

 the cross-section shows 2-4, or even more concentric layers of apparently similar, 

 tangentially flattened cells, and only very accurate investigation teaches that these are 

 non-equivalent, one being always the initial layer, while the others are partly tissue- 

 mother-cells, partly young wood or young bast. The latter fact often appears especially 

 clearly on the side of the wood, when growth begins anew after the winter's rest, for 

 then certain of the cells are found undergoing extension to form members of vessels, 

 directly and without further divisions (e. g. Vitis vinifera). Thus, even during the 

 winter's rest, the layers of the true cambium are not distinguished either among 

 themselves, or from the young bast and young wood by any characteristic structure ; 

 on the contrary, the entire zone of secondary growth, whether consisting of all its 

 possible parts, or of the cambium, or the initial layer alone, may enter upon the 

 "winter's rest, and then assumes everywhere the same structure. 



Having finished the description of the zone of secondary growth, we still have 

 to return to the transverse and oblique cell-divisions taking place in it, which were 

 left unexplained above. In the medullary rays these do not occur, or, if they do, are 

 irrelevant. In the elongated elements, on the other hand, they appear as a constant 

 and essential phenomenon. 



(a) In the tissue-mother-cells they appear universally where short parenchyma- 

 tous cells which do not belong to the medullary rays, and septate fibrous cells, are 



H h 2 



