SECONDARV THICKENING. NORMAL DICOTYLEDONS. 46 1 



large medullary rays, or in some of them, small intermediate bundles subsequently 

 appear, which have an undulating course K Here, however, the process appears to 

 be rare ; I could not find it in my material, even in stems an inch thick. In the 

 broad medullary rays of the wood of Casuarinse the formation of secondary inter- 

 mediate bundles seems to occur constantly, and therefore, strictly speaking, separates 

 the stems in question from the normal Dicotyledonous type'^ The inner, oldest 

 portion of these bundles extends vertically, without interruption, through the whole 

 internode. After some years intermediate bundles appear further outside, which are 

 at first very small, but constantly become thicker as secondary growth goes on, and 

 these frequently anastomose with one another and with the neighbouring main bundles, 

 in an irregularly undulating course. In the older wood, even in branches one inch 

 thick, the space corresponding to the original medullary ray is divided up by an 

 irregular net, with pointed meshes, of small bundles. Menispermum canadense pre- 

 sents the same phenomenon in a less conspicuous degree. To what extent similar 

 bast-bundles, corresponding to the secondary bundles of wood may occur, still 

 remains to be investigated. 



Sect. 136. When once formed, the cambial ring constantly increases in thick- 

 ness and circumference, while successive reciprocal differentiation of wood and bast 

 goes on. The growth of the transverse diameter of its individual cells takes place 

 in a much smaller degree than the general growth, though in many cases they show 

 an increase within definite limits. On the other hand, a continual increase of the 

 number of cells goes on, by division of the existing ones. The general growth, the 

 divisions, and the ditferentiation of the products of the division in the direction of 

 wood and bast, proceed during the periods of active vegetation ; they cease during 

 the periods of rest in winter. The course of the successive divisions and differentia- 

 tions is obviously only to be determined by investigation during the period of 

 vegetation, especially at its commencement. In order to test and confirm the result 

 thus obtained, a comparison of the resting stage in winter is of service. 



In order to explain the general course of the divisions, we may, in the first 

 instance, consider the transverse section alone, and assume 'that the cambial layer, 

 as well as the immediately contiguous youngest layers of wood and bast, consist of 

 entirely similar cells, ranged in radial rows ; this assumption applies with tolerable 

 exactness in the case of the bundles of the wood. 



The following rule was first established by Sanio ' in the case of Pinus sylvestris. 

 (See Fig. 195.) All cell-division proceeds from a single, annular, meristematic layer 

 of cells, one row thick as seen in cross-section, which may be called the Initial layer. 

 In this every (initial) cell divides by a tangential longitudinal wall into two daughter- 

 cells, one of which once more becomes an initial, while the other becomes a tissue 

 mother-cell ; the latter change may happen either to the inner of the two daughter- 

 cells, which is added to the wood, or to the outer, which is added to the bast. In 

 the quite regular case of Pinus sylvestris, each tissue mother-cell then divides once 

 by a tangential wall, and the two products of its division become directly converted 



' Compare Sanio, Botan. Zeitg. 1863, p. 127. 



' Gbppert, LinnEea, XV. (1841), p. 747, Taf. IV. Fig. 7.— Sanio, I.e. 



' Pringsheim's Jahrb. Bd, IX. 



