468 



SECOND A RV CHANGES. 



formed within the secondary wood and bast; wood-parenchyma, bast-parenchyma, &c. 

 (Fig. 202.) Tlie transvei-sc divisions take place once or oftener, and accordingly the 

 height of the products of division is unequal from the first ; from the different inser- 

 tion of the transverse walls on the lateral walls, and the relative thickness of the two 

 in the mature tissue, it may be conjectured that the transverse divisions take place in 

 the one case in the early condition of the tissue-mother-cells (wood and bast paren- 

 chyma), in the other case in cells already belonging to the young wood or young 



bast (septate fibrous cells) ; on this point, however, more 

 exact investigations have not as yet been made. Comp. 

 below, Sect. 144. 



Transverse divisions of the tissue mother- cells, or 

 of the young wood-cells, are found in some instances 

 before the formation of vessels with short members. 

 The members of wide vessels with horizontal limiting 

 surfaces may be shorter than the cambial cells from 

 which they arise ; the difference in length may, however, 

 be due to the fact that, as they become wider, their height 

 is diminished by displacement of the oblique terminal sur- 

 faces into the horizontal position \ In other cases how- 

 ever, e. g. in that of Vitis observed by Cohn '^, and in that 

 of Acacia longifolia observed by Sanio ', the shortening of 

 the members of the vessel is so considerable that it does 

 not find a sufficient explanation in this displacement, but 

 rather necessitates the supposition of a transverse division. 

 {b) In the initial layer those transverse and oblique di- 

 visions of the elongated cells have to take place, by means 

 of which new small medullary rays, consisting of short 

 parenchymatous cells, originate within the bundles of the 

 wood. This follows almost with certainty from the fact 

 that the medullary ray, from its first appearance onwards, 

 extends through the initial layer, towards wood and bast. 

 The only other possible supposition would be that its 

 first origination is due to divisions which extend, in the 

 same direction, through cambium, young wood, and young 

 bast. The first formation of a small medullary ray from 

 elongated cambial cells is hard to observe, and is at present not very clearly 

 known*. From the position of very small medullary rays, only one cell in breadth, 

 and one or a few cells in height, as seen in tangential sections through the cambium 

 and the zone of secondary growth, it may be stated that they originate either by 

 single or repeated transverse division of an end of an elongated cambial cell, or 

 by the cutting off of a portion of the radial lateral wall of the latter, by means 

 of a (mussel-shaped) wall of division, with its concuve side turned towards the 



Fir;. 202.— Cytisus Laburnum ; tan- 

 gential longitudinal section through 

 the innermost layer of bast of the 

 same branch as Fig. 198; magnified 

 as in the latter, j^ members of sieve- 

 tubes ; t a sieve-plate lying deeper 

 than the surface of section ; w small 

 medullary ray, two cells high. The 

 renia*ning elements are parenchy- 

 matous cells of the bast, the origin 

 of which by transverse division of 

 the cambial cells becomes clear on 

 comparison with Fig. 198. 



' Sanio, Botan. Zeitg. 1863, p. 122. 



^ Bericht liber d. Verhandl. d. Schles. Gesellsch., Bot. Section, 1857, p. 44. 



' Pringsheim's Jahrb. IX. p. 56. 



* See N. Miiller, Bot. Untersuchungen, IV. p. 181.— Velten, Botan. Zeitg. 1875, p. 842. 



