804 Transactions of the Society. 



found on the inner side of the cambium in this species, a few cells 

 of small calibre, which do not appear to have been developed from 

 it; although I have not been able to trace their origin with 

 certainty, the probability being that they result from the division 

 of the outer cells of the medulla. These cells may remain per- 

 manently thin-walled, or some of them may become thickened — a 

 difierentiation, however, which is not observed until the cambium is 

 some three or four cells in thickness. 



With regard to the order of development of the cells forming 

 the cambium, it was just stated that in the earliest stage observed 

 in the laburnum, there were two tangential septa thrown across the 

 mother-cell, and that the one nearest the phloem being the thin- 

 nest, was the youngest. In the consideration of this part of 

 the question we are greatly helped by a study of what takes place 

 in Ficus carica. In this species the process of development is 

 somewhat modified, as a reference to the figures will at once render 

 apparent. Fig. 6 is a section through the procambium imme- 

 diately after its difierentiation into phloem elements, and no indica- 

 tion of either xylem or cambium is seen. In Fig. 7 at a we find 

 a cell adjoining the medulla, which has enlarged and divided 

 tangentially into two daughter-cells. At h, h we notice that in two 

 cells in which this same process commenced earlier the daughter- 

 cell next the pith has become spirally thickened, while its sister- 

 cell has grown and divided again in the same direction. Growth 

 in the two daughter-cells at first takes place equally, so that we find 

 them both of one size ; but there is this difi'erence in them, that 

 while one passes over into a permanent form, the other preserves its 

 generating character ; so that by the time the one has become fully 

 developed the other is again ready for further division. 



It will be observed that the cells we have just been dealing with 

 are separated from each other by one or two cells of smaller dimen- 

 sions. In these, growth takes place equally with those already 

 described, but the divisions arise irregularly, sometimes being tan- 

 gential, at others radial. 



This order of development continues till the condition shown in 

 Fig. 8 is reached. Here we have, proceeding from the centre of 

 the stem, a series composed of two or three spiral cells or vessels 

 preceded by two or three thin- walled cells, the whole of them being 

 produced in regular succession by division of the cell immediately 

 adjoining the phloem. Thus far, then, development has taken 

 place in regular progression, proceeding from the centre outward. 

 At this stage we notice that tangential septa arise in the cells 

 which separate these series of thickened cell-forms the one from 

 the other, and thus a continuous band of cambium is produced in 

 the bundle. 



At first sight it would appear that in this species we have an 



