24 THE PHYSIOLOGY OF STOMATA. 



the air by solution (Copeland, 1902). Furthermore, stomata, when mounted 

 for observation in water, may, as others, including myself, have found, suffer 

 sudden changes in contour (von Mohl), a possibility fatal to my object. 

 Indeed, I found that invariably the stomata of Verbena ciliata suddenly 

 enlarge their transverse (tangential) measurements when water reaches them. 

 This may be determined without very great difficulty if a piece of epidermis 

 is mounted* for microscopic observation first without the addition of any 

 medium. After observing the size of certain stomata, these, upon running 

 water under the cover, will be found to open wider. Thus, 5 closed stomata 

 in different pieces of epidermis taken at 6 p. m. opened so that their pores 

 became 6 micra broad. In another case, the outside transverse measure- 

 ment of 4 neighboring stomata, all of which were closed, were taken. These 

 were, in micra, 28, 26, 24, and 27. Upon the addition of water, the measure- 

 ments were 34, 32, 28, and 30, respectively. 



By means of a stop-watch the rate of opening may be determined with but 

 fair though sufficient accuracy. Thus a stoma opened o to 7 micra in some- 

 what less than 3 seconds. It will be appreciated that, to mount epidermis 

 in water and, after placing on the stage, then to examine the stomata, might 

 result in hopeless error. Nothing can be said of stomata mounted in water, 

 as regards their condition in situ, unless it is known that placing in water 

 does not effect them in the least, which is unlikely in any case. 



Again, the removal of epidermis has been objected to on the ground that 

 this results in a release of tensions which are responsible in part for the size 

 of the stomatal openings. I have suspected this to be true in the case of a 

 plant, Chrysoma pauciflosculosa (Lloyd, 1901), which has a very peculiar leaf 

 anatomy, though it may be possible that a local shrinkage of the thickness 

 of any flattened leaf, as might occur in the areas between the vascular tissues 

 during wilting, may effect the ratio between the size of the outer and inner 

 opening of the stomata. This, however, is a different case from a general 

 tangential stretching of the epidermis due to the turgidity of the leaf. There 

 is, quite probably, upon removal of a piece of epidermis, a slight shrinkage of 

 general dimensions. The effect, it would seem, upon the condition of the 

 stomata, must be exceedingly small, so that the differences before and after 

 removal would be inappreciable. 



The upward curving of the leaf of Verbena ciliata (see beyond, p. 81) during 

 the early stages of wilting implies that the upper epidermis is reduced in tan- 

 gential dimensions, but this is only in a very small amount. If we assume 

 a shrinkage of 5 per cent during this time, a very liberal estimate, it would 

 mean in a single stoma a corresponding percentage of shrinkage. A stoma 

 widely open measures, say, 10 micra across the pore. This would mean a 

 reduction in size from 10 to 9.5 micra, a scarcely measureable amount, and 

 within the personal error of observation. In the absence of special structural 



*On account of the numerous hairs, epidermis of Verbena should in this operation be 

 viewed from the under side. 



