620 



LECTURE XXXV. 



from the sunlight and exposed merely to diffused daylight, the chlorophyll- 

 corpuscles abandon the above position and spread themselves on the outer walls of 

 the cells. The changes in position described can be called forth at will, and 

 often by alternately intense and feeble illumination. 



Of Stahl's numerous observations on the chlorophyll-tissue in the leaves of 

 Phanerogams we may here refer further to his statements with respect to Oxalis 

 Acetosella (Wood- sorrel). ' The cells of the uppermost layer next the epidermis 

 are developed into more or less obtuse cones, the bases of which are situated 

 on the epidermis. The two lower layers of the mesophyll consist of flat 

 stellate cells as in Fig. 363, Healthy leaves of this plant were laid flat on 

 a plate and exposed to the rays of the sun falling perpendicularly on them. By 

 pouring fresh water over them the leaves were prevented from becoming too 

 warm. Individual leaflets were protected from the direct rays of the sun by 

 means of paper shades. After an hour the marked leaflets were placed in 

 alcohol, in order to fix the cell-contents in their position. The decolorised leaflets 

 were so transparent that mere observation with transmitted light was sufficient to 

 demonstrate the diflferent distribution of the chlorophyll-corpuscles in the shaded 



FIG. 363.— Cells of the lo\ 

 direction at right angles to t 

 liijht ; b profile position after a 



•erniost layer of spongy parenchyma from the leaf of Oxalis acetocella, seen in a 

 le surface of the leaf a plane position of the chlorophyll-corpuscles in diffuse 

 short exposure to the sun ; c after longer insolation (after Stahl). 



leaflets and those exposed to the sun. Fig. 363 a shows the surface view of a 

 stellate cell in a shaded leaf; the chlorophyll-corpuscles are distributed approximately 

 uniformly on the walls parallel to the surface of the leaf. In Fig. 363 h 

 the grains have passed over on to the walls which are perpendicular to the 

 surface of the leaf ; this distribution is found after insolation which has not 

 been continued too long. If the leaves have had the sun shining on them for 

 a longer time — an hour or more — the arrangement of the chlorophyll represented 

 at c is met with ; the corpuscles are collected into clumps, lying on the walls common 

 to two neighbouring cells. 



I may here interpolate, with reference to what has been said above as to 

 the alteration in colour of the leaves in intense and in feebler light, that it is 

 now easily intelligible from the examples given, how the different positions of the 

 chlorophyll- corpuscles must effect alterations in the intensity to the unaided eye of 

 the green colour of the leaves ; it is obvious that a leaf, the chlorophyll-corpuscles 

 of which all lie on the side walls of the cells, must appear to the eye of a brighter 

 green than one where they lie on those walls which are parallel to the surface 

 of the leaf 



With respect to the mechanics of these phenomena, I have already expressed 



