1883.] development of the Leaves of Pinus Silvestris L. 347 



prominent points in the several stages succeeding this, until at 

 length the guard cells appear to lie as much in the hypodermal 

 row as in the row from which they were originally derived, i.e. 

 between the epidermal and the hypodermal cells, and are only 

 separated by a slight space from the peripheral layer of the cortical 

 ground tissue. The two hypodermal cells which they have dis- 

 placed now lie partly beneath and partly at the sides of the guard- 

 cells. The latter are now so much depressed below the surface 

 that they are separated from it by an elongated oblong funnel, pit, 

 depression or fossa forming an ante-chamber which is bounded 

 laterally by the four complemental cells of the group under the 

 inferior borders of which they are let in. These complemental or 

 accessory cells have in their growth and development kept pace 

 with the other cells of the epidermis as they do in Salisburia and 

 consequently are of large size : their upper free extremities have 

 also converged slightly towards each other over the funnel (since 

 the pressure which previously existed has been relieved in that 

 direction), and have become somewhat rounded 1 . 



In longitudinal sections, and when viewed from above, the guard- 

 cells have an elongated, crescentic form. The walls of the epidermal 

 cells, which have assumed in the meantime a fine irregular wavy or 

 sinuous outline, though still preserving their original polyhedral 

 form, now become in great part cuticularized, and this is especially 

 the case on those borders of the four complemental cells belonging to 

 each group which are towards the ante-chamber and are excessively 

 thickened ; while those of the hypodermal cells become sclerenchy- 

 matous. Thickening, accompanied by cuticularization of the wall 

 separating the two guard-cells, also takes place, but unequally, the 

 thickening being greatest in the middle and more pronounced 

 towards the internal than towards the external side. In the next 

 stage the two hypodermal cells have come to lie entirely laterally, 

 the wall between the two guard-cells then splits down the middle, 

 and the guard-cells separate from below upwards diverging as they 

 do so but still remain attached at their outer ends. They alone 

 contain chlorophyll grains, and are thus well marked off from the 

 other epidermal cells. The last stage of all consists in the complete 

 separation of the guard-cells from each other to form the cleft and 

 the adult state of the stoma (which alone has hitherto been figured) 



1 The funnel is thus slightly contracted in the upper part by the projection of the 

 epidermal cells, expanded in its middle part and again contracted slightly at the 

 base above the guard-cells. Its depth is not very great as compared with those in 

 the leaves of other Conifers, since the hypoderma is only at most two cells deep, 

 and as Hildebrand has shown, a distinct and definite relation exists between the 

 depth of this layer and that of the funnel above the guard-cells. 



The upper walls of these four complemental cells moreover lie on a perfect level 

 with the general surface of the epidermis, and are not in any way either depressed 

 beneath it or elevated above it. Thus the stomata belong to Hildebrand's second 

 type (loc. cit.), which is the most frequent one in the Conifera. 



VOL. IV. PT. VI. 25 



