SECONDARY THICKENING. NORMAL DICOTYLEDONS. 



465 



The structure of the cambium, and of the young wood and bast, has been given 

 above, as regards the arrangement of the cells when seen in cross-section. 



In each portion of the cambial zone, the form of the cells is identical with or 

 similar to the general form of the elements of that section of the mature wood and 

 bast which borders on it in the radial direction. (Figs. 198, 199.) Parenchymatous 

 medullary rays are bordered by portions of the cambium, in which the shape of the 

 cells is almost identical with that in the rays themselves, except that the relative 

 radial diameter is on the whole smaller ; while the bundles of wood, and the rare 

 medullary rays composed of fibrous cells, are bordered by cambial cells having the 

 form of elongated fibrous elements. Regarded more 

 minutely, the form of these elongated cambial cells is 

 very uniform in all the known cases. It was first 

 accurately recognised by A. Braun^, though only in- 

 dicated by him, and has lately been fully described by 

 Velten ^. It is that of a rectangular prism, of which the 

 radial transverse diameter is smaller than the tangential 

 (on the average about half as long), while the ends of 

 it, owing to an inclination of the radial lateral walls 

 to the radial plane, form a sharp edge, lying radially and 

 almost horizontally. The inclination of the lateral walls 

 is usually confined to one side, and then directed alter- 

 nately to the right and left, more rarely (e. g. Caragana 

 arborescens, Cytisus Laburnum) the two radial surfaces 

 are inclined to one another like a roof. The steepness 

 of the inclination varies, partly according to the individual 

 case, partly, in its average degree, according to species; 

 and is on the whole the greater, the more the cells are 

 elongated; the relatively short cambial cells of Tilia 

 parvifolia, for example, have their end-surfaces inclined 

 at about 45°, the relatively very long ones of Hamamelis 

 virginiana are quite gradually tapered and pointed, as 

 seen in the tangent^al view. Very slightly inclined or 

 horizontal terminal surfaces ' only occur in isolated cases, 

 especially above, or at the side of medullary rays. (Fig. 

 199.) From the conditions of form described, it follows that the elongated cambial 

 cells, which, as meristem, are in uninterrupted contact, also form, as a rule, un- 

 interrupted longitudinal rows, and only form alternating horizontal rows in the case 

 of uniform inclination of the two radial surfaces. 



The absolute average size of the cambial cells varies according to the species, as 

 will be stated below (Sect. 153). In certain cases it remains on the average the same 

 during the whole growth in thickness, or increases continuously for a series of years, 

 until a size is attained which remains approximately constant. And in fact this 



Fig. 198 — Cytisus Laburnum 

 three -year- old branch, during the 

 winter's rest (March) ; tangential sec- 

 tion (145). acbd'C[v& zone of secondary 

 growth and cambium, containing a 

 medullary ray above,and bordering on 

 the autumn wood, A, of the previous 

 year. 



^ Monatsber. d. Berliner Acad. 7 Aug, 1854, p. 50 d. Sep. Abdr. Anmerkg. 

 2 Botan. Zeitg. 1875, p. 811.— Compare also Sanio, Botan. Zeitg. 1863, p. lo8.- 

 Eot. Unters. Heft IV. 



Hh 



-N. MuUer, 



