

IN THE .MOTOR ORGANS OF LEAVES. 



41 



differing, however, from common epidermal tissues, inasmuch as all the constituent cells, 

 with the exception of those of the marginal row bounding the frond and connected 

 with both strata, are abundantly provided with chlorophyll, the cells of the superior 

 stratum especially containing a variable number of relatively lar«re chromatophores 

 (plate I. fig. 6). The superior epidermis over the greater part of the frond consist* 



of short polygonal cells, with wavy margins, and with numerous narrow pits or actual 



on the external or upper face of their walls (plate I. fig. 3). Towards the 



slits 



margins of the frond, the cells become somewhat more elongated, and the outer ones 

 are in direct contact with the common marginal row, to which the external part of 

 the inferior epidermis is also attached. The marginal cells are, as before said, devoid 

 of chlorophyll and are of a narrow, elongated figure, some being prolonged antcri. arly 

 into projecting points which give the frond a serrated contour (plate I. fig 

 Their walls are greatly thickened and highly cuticularised, especially externally, and 

 the projecting points are almost completely filled up so as to form solid spines. 



The inferior epidermis, except immediately over the midrib, is composed of 

 elongated cells with strong walls and containing only a comparatively small quantity 



of chlorophyll (plate I. figs. 1, 2). A few stomata are present along the middle line 

 of the superior epidermis, and a good many occur generally diffused over the inferior 

 one. The inferior epidermis and the highly thickened rim form a sort of ri^id 

 frame on which the superior epidermis is highly stretched in such a fashion that, 

 even were its cell walls extensile and elastic, any considerable alteration in the 

 capacity of the cell cavities is rendered impossible under ordinary circumstances. 

 Between the two epidermal strata there is a thin layer of very open tissue, which 

 is somewhat thickened along the middle line around the single vascular bundle, but 



which thins out and disappears around the edges and distal extremities of the leaflets. 

 Each leaflet thus consists of a comparatively rigid inferior stratum, a very resistent 

 margin, a superior stratum rich in chlorophyll and consisting of cells the walls of 

 which are pitted towards the free surface, and a limited amount of loose intermedial e 

 tissue including a vascular bundle. 



The processes which determine the conspicuous changes in the colour of the fronds 

 take place in connection with the cells of the superior epidermis. When we examine a 

 series of preparations of this derived from fronds in maximal conditions of greenness 

 and whiteness, it becomes at once evident that the protoplasmic constituents of the cells 

 are in very different states in the two cases. In the bright green diurnal condition 



(plate I. figs. 4, 6), the protoplasts entirely fill the cell-cavities and the separate 

 chromatophores which they contain are more or less distinctly recognisable; whilst, in 



the maximal white state (plate I. figs. 5, 7, 8), the protoplasm is shrunken and con- 

 tracted away from the cell walls, and the chromatophores are so closely packed together 

 as to form a spherical mass in which the individual elements are irrecognisable, and 

 which in many cases is the only visible representative of the protoplasmic content of 



1* 



the cell. In other cases a narrow, irregular fringe of colourless, granular protoplasm, 

 containing a nucleus, may be detected around more or less of the periphery of the 

 chromatophoric sphere. Between the outer margin of the protoplasm and the cell-wall a 

 wide space intervenes, which in the natural state of the tissue appears to bo occupied 

 by air, and it is to the presence of this intracellular air that the peculiar silvery or almost 

 chalky whiteness of the fronds is due. If, finally, we examine fronds in the green state 



hich they assume during the latter part of the night (plate I. fig. 9), we find the 



Rot. Bot. Gard. Calcutta Vol. YI 



