VIII] OF STOMATA 627 



its margin loop from rod to rod in curves which are possibly 

 catenaries but are more probably portions of an "elastic curve," 

 and the outward resemblance to a Pluteus larva is now complete. 

 By various slight modifications, by altering the relative lengths of 

 the rods, by modifying their curvature or by replacing the curved 

 rod by a tangent to itself, we can ring the changes which lead us 

 from one known type of Pluteus to another. The case of the 

 Bipinnaria larvae of Echinids is certainly analogous, but it becomes 

 very much more compHcated; we have to do with a more complex 

 partitioning of space, and I confess that I am not yet able to 

 represent the more compUcated forms in so simple a way. 



There are a few notable exceptions (besides the various unequally 

 segmenting eggs) to the general rule that in cell-division the mother- 

 cell tends to divide into equal halves ; and one of these exceptional 

 cases is to be found in connection with the development of 

 "stomata" in the leaves of plants*. The epidermal cells by which 

 the leaf is covered may be of various shapes; sometimes, as in a 

 hyacinth, they are oblong, but more often they have an irregular 

 shape in which we can recognise, more or less clearly, a distorted 

 or imperfect hexagon. In the case of the oblong cells, a transverse 

 partition will be the least possible, whether the cell be equally or 

 unequally divided, unless (as we have already seen) the space to 

 be cut off be a very small one, not more than about three-tenths 

 the area of a square based on the short side of the original rectangular 

 cell. As the portion usually cut off is not nearly so small as this, 

 we get the form of partition shewn in Fig. 276, and the cell so cut 

 off is next bisected by a partition at right angles to the first; this 

 latter partition splits, and the two last-formed cells constitute the 

 so-called "guard-cells" of the stoma. In other cases, as in Fig. 277, 

 there will come a point where the minimal partition necessary to 

 cut off the required fraction of, the cell-content is no longer a 



* We know more about the physical activities of the storaata than about the 

 mechanics of their development. It is known that the rate of gaseous diffusion 

 through apertures of their order of magnitude is inversely proportional to the 

 diameters of the apertures; and this law, by which the sufficient entry of carbonic 

 acid through the stomata is fully accounted for, is (like Pfeffer's work on natural 

 semi-permeable membranes) one of the notable cases where physiology has enlarged 

 the boundaries of physical science. Cf. Horace T. Brown, Some recent work on 

 diffusion, Proc. Roy. Instit. March, 1901. 



