MorpJiology of Vegetahle Tissues. By W. H. Gilhurt. 807 



the fact, that in cutting a series of sections of the whole region 

 referred to, it will be found, after leaving the parenchyma and soft 

 bast, we find nothing but groups of cells as described, until the true 

 wood is reached. Moreover, on the side next the phloem we find the 

 cambium passing over to parenchyma by the rounding off or fui-ther 

 division of the individual cells, and on the xylem side we find the 

 transverse septa absorbed, not always wholly or at once, for traces 

 of them are often to be seen on the cell-walls even after thickening 

 has commenced. 



Thus in Cijtisus lahurnum, a species well adapted for study, it 

 will be seen in Fig. 9, which is taken from a section adjoining the 

 parenchyma of the bast, the cells are comparatively broad ; that in 

 Fig. 10, which is from about the centre of the cambium, they are 

 narrower and proportionately longer, while the two combined cells 

 are more decidedly spindle-like in form ; and in Fig. 11, they have 

 been still further modified in the same direction, the dividing trans- 

 verse septa having disappeared — this figure being taken from a 

 section nest the xylem. 



The same process in every respect is illustrated in Figs. 12 and 

 13, drawn from sections of Syringa vulgaris. Here the groups 

 are composed of three and occasionally four cells. The modifica- 

 tions take place in exactly the same order ; decrease in diameter, 

 increase in length, and absorption of septa. 



If we look at the development of the cambium in the other 

 direction, or on the phloem side, we should observe m Cytisus that 

 as a rule each cell divides into two ; and so we sometimes see on 

 the outside of the tissue, groups with spindle-like ends composed of 

 four cells ; but growth very speedily obliterates the form, and 

 intercellular spaces arising, we have a true parenchymatous tissue.* 



Such then being the character of the cambium, it follows, that 

 we can no longer regard prosenchyma as being composed of 

 differentiated original elementary cells, for we find that they are 

 like the vessels, but in a much more limited sense, cell-fusions. 



This view of the structure of the cambium also involves of 

 necessity a modification in the account given of the origin of other 

 elements of the xylem, viz. the wood-parenchyma. 



Sachs says t " They arise (i. e. the parenchymatous cell-forms 

 of the xylem) according to Sanio by transverse division of the 

 cambium-cells before their thickening commences." From this it 

 would appear that Sanio saw these cell-groups in the cambium, 

 but from some cause failed to observe that the whole of the 

 cambium was composed of them. Sachs also figures these paren- 



* With regard to the origin of the fibrous elements of bast, tliey appear to arise 

 in two ways, viz. the combination and fusion of the cambium cell-groups, and also 

 by the fusion of vertical series of parenchymatous cells. This point, however, has 

 not been satisfactorily made out, and is reserved for further observation. 



t Sachs' ' Text- Book of Botany,' p. 100, Oxford, 1875. 



