1883.] development of the Leaves of Pinus Silvestris L. 357 



resin-wall at the time when they separate, those cells which compose 

 the circle exterior to them are pressed upon from within and each 

 usually divides tangentially by a wall into two segments. The cells 

 of the resin- wall themselves also divide tangentially and their more 

 external segments are added to the outer circle. Each cell of the 

 resin-wall now becomes divided in turn by a radial wall, so that six 

 and then eight cells result, which then round themselves off so that 

 the resin passage becomes larger. Some or all of these may divide 

 again radially so that there are sometimes twelve, fourteen, or six- 

 teen cells in the completed resin-wall. The cells of the resin-wall 

 form thus a sort of membrane or secreting epithelium only one cell 

 deep around the cavity of the resin passage, which latter has a 

 schizogenous mode of origin. While the cells of the resin-wall are 

 being divided radially, divisions in a similar plane also occur in the 

 numerous cells lying exterior to them which have also been derived 

 from the primitive mother-cell : these divisions enable the cells of 

 the outer circle to keep pace with the increasing dimensions of the 

 resin-wall within. The number of rows of cells in the external 

 circle bears however a strict relation to the ultimate number of cells 

 forming the resin-wall: thus if the four daughter-cells which consti- 

 tute the primitive resin- wall divide only a small number of times, 

 i.e. so as to form only six or eight cells in all (a not unfrequent 

 occurrence), then radial division of the cells of the outer circle also 

 ceases early and they begin to increase by tangential walls instead, 

 forming not one but two or three circles of cells. If on the other 

 hand the cells of the primitive resin-wall divide radially a large num- 

 ber of times so that there are as many as fourteen or sixteen cells in 

 all, then the cells of the outer circle have little time to divide tangen- 

 tially and hence form only a single row. In the former case the cell- 

 walls of the cells forming the outer circle become greatly thickened 

 and sclerenchymatous, so that their cell cavities are rendered very 

 small or may disappear completely, their cell contents disappear 

 early, and they form a firm protective sheath or investment for the 

 resin passage and the resin-wall, giving rigidity to them in order 

 that the cavity of the passage may not be closed up by pressure 

 from the midrib or other prominent portions of the leaf. They are 

 not however in any way hypodermal cells as Mons. C. E. Bertrand 

 (loc. cit.) and Dr Maxwell T. Masters seem to regard them 1 . In the 

 latter case however the walls of the cells of the outer circle never 

 have time to become greatly thickened before the completion of 

 the resin-duct. So much can be made out by means of transverse 

 sections. Since however to each cell of the group of four daughter- 

 cells a longitudinal row of cells lying one over the other corresponds, 



1 Vide "On the Belations between Morphology and Physiology in the Leaves 

 of certain Conifers," Jour. Linn Soc. Bot., Vol. xvn. No. 105, pp. 547 — 552, March 

 1880. 



