802 Transactions of the Society. 



The doctrine generally held concerning the origin of the 

 cambium may be briefly stated as follows : — 



That after the development from the primary meristem of 

 the primary cortex and pith, a zone remains Ijetween them, 

 ■which preserves its merismatic character, and is called the pro- 

 cambium ; the cells of which are similar to each other, and are 

 combined without intercellular spaces ; that on either side of this 

 procambium the cells are differentiated into permanent tissue, on 

 the outer side bast and parenchyma, on the inner side prosen- 

 chyma and vessels; and that in open bundles a certain portion 

 remains over between these two tissue systems ; this portion con- 

 stituting the cambium layer. 



Now in the sections through the apices of the stems and 

 terminal buds of certain trees referred to above, I observed that on 

 the inner side of the procambium ring or bundles, there were a 

 greater or less number of cells which appeared in every respect to 

 agree with those which in older stems separate the sylem from 

 the phloem; being thin-walled, regular in disposition, without 

 intercellular spaces, and arranged radially with regard to the axis 

 of the stem. Examming a section somewhat farther from the 

 apex, I found that a few of the cells adjoining the pith were 

 thickened; while in a section nearer to the apex, the procambium 

 zone was thinner, and the cambium-like series absent. 



The thought naturally suggested by these observations was, 

 that possibly the cambium was not in itself a portion of the pro- 

 cambium, but a special tissue developed from it. In order to 

 determine this, I have made many preparations of the terminal 

 buds of some sixteen species, but owing to the sharply curved 

 course taken by the procambium at or near the apex, and conse- 

 quently the line of section being at a more or less acute angle with 

 its length, the inquiry proved one of considerable difficulty. 

 Nevertheless, in all cases at a suitable distance from the apex, this 

 cambium-like tissue was to be found either with or without one or 

 two thickened cells on its inner surface, while the remainder of 

 the procambium was undergoing differentiation into hard and soft 

 bast. 



A few species, however, were most instructive, especially the 

 Laburnum (Oijtisus laburnum), Lombardy poplar (Pojmhis nigra), 

 and the Fig (Ficus carica) ; and after a careful and prolonged 

 study of them, there seems but one conclusion possible ; viz. that 

 the whole of the procambium is differentiated into hard and soft 

 bast, with the exception of its innermost layer, the cells of which 

 by means of successive tangential divisions, produce a cambium 

 tissue ; this new generating tissue preceding in order of develop- 

 ment the appearance of any thickened cell-forms on the xylem 

 side. 



