198 Mr J. M. Macfarlane on the 



divided, and by the time that complete separation of the 

 two cells of the stoma has been brought about, the nucleolus 

 also may have divided, giving us two nucleoli in each guard- 

 cell, and this condition may be permanently retained. This 

 may seem an unimportant fact, but it helps us to make a 

 broad generalisation as to division of the cell generally, viz., 

 that the nucleolus, or more probably the nucleolo-nucleus, 

 is the centre of germinal activity, and that as we pass out- 

 wards to the periphery of the cell, this reproductive activity 

 becomes less and less. In no other way, to my mind, can 

 the number of nucleoli and nucleolo-nuclei at different ages 

 in the cells of any plant be explained ; but regarding this 

 as the true explanation many difficulties vanish. If such, 

 then, be the case, we should expect to find that, occasion- 

 ally at least, each guard-cell nucleus should split up, and, 

 by formation of a septum, give rise to two cells. Were 

 this morphological change to take place, the physiological 

 function of the stoma would necessarily be destroyed. This 

 interesting pathological change has thrice come under my 

 notice. Ordinarily, as the guard-cells become aged, the 

 nuclei may get shrivelled, and protoplasm, with chlorophyll 

 bodies and starch, greatly fills the interior. They would 

 seem in truth to have played out their important function. 

 The whole of the above changes have also been seen in 

 SciUa hifolia, except the strongly marked dumb-bell shape 

 assumed by tlie nucleolus before division. Let us return 

 now to the epidermal cell. When newly cut off it exactly 

 resembles the stoma mother-cell, but is soon distinguished 

 by the great elongation which follows. In some speci- 

 mens a very pretty alternate arrangement of epidermal 

 and stoma cells, on the principle, so to speak, of the nodal 

 and internodal cells of Chara, prevails over the surface, 

 except where fibro-vascular bundles traverse the leaf. If 

 this be the fundamental constitution it is only exception- 

 ally met with, as, by repeated transverse division of the 

 epidermal cells, two or more of these may intervene 

 between two stomata. These divisions are carried out in 

 exactly the same way as in the stoma, only that nucleus, 

 nucleolus, and, I should imagine also, nucleolo-nucleus, 

 elongate, not parallel, but at right angles to the former 

 plane of division, the new partition running, not as in the 



