82 THE PHYSIOLOGY OF STOMATA. 



bilis (Lloyd, 1902), and the inrolling on grass leaves may be regarded as a 

 similar movement, clearly due to mechanical conditions resident in the 

 tissues. 



Though the experiment is very simple it nevertheless helps one to form a 

 conception of what goes on in the verbena leaf during wilting.* It seems 

 probable that during the first few minutes the loss of water results in a general 

 lowering of tensions produced by high turgidity, the particular kind of move- 

 ment of any leaf being one brought about as the result of its particular arrange- 

 ment of tissues. This lowering of tensions means also a squeezing together 

 of the cells and a reduction of the inter-cellular spaces, which again will result 

 in the sudden forcing out of the contained gases, and therefore water-vapor. 

 We should, then, find a sudden increase in the rate of transpiration, if we may 

 call it such, due to a mass movement of water- vapor, and not necessarily a 

 change in the condition of the stomata. A change of this kind is purely a 

 mechanical matter, and can hardly be regarded as of any significance. 



Having satisfied himself that the temporary opening occurred, Darwin 

 sought to determine whether the degree of this opening differed in the morn- 

 ing and afternoon, and he obtained positive results, chiefly with Poinsettia, 

 which showed a temporary opening in the morning but none in the afternoon. 

 The explanation of this difference Darwin referred to the induced periodicity 

 of the stomata, reference to which has been made elsewhere (p. 74). Assum- 

 ing Darwin to be perfectly right in his belief that a temporary opening occurs, 

 it is possible that the movement of the guard-cells is purely a mechanical 

 result of the lowering of tissue tensions in the epidermis, and not a physio- 

 logical process having anything in common with the different responses 

 obtained by exposure to light or darkness in the morning and afternoon, 

 though the differences in turgor in morning and afternoon may be common 

 factors in both. This assumption, while justified upon speculative grounds, 

 does not seem in this matter to lead to a definite conclusion. What is essential 

 is to determine without doubt whether such a temporary opening occurs or 

 not, and this I have tried to do. Two methods have been used — direct 

 observations through the microscope of the living stomata, and the measure- 

 ments of stomata after fixation in absolute alcohol at suitable intervals. In 

 the case of ocotillo the latter method alone has been used. A sufficient 

 number of leaves were removed from the plant and the epidermis from one of 

 them fixed at certain periods. In the case of Verbena, in which the leaves 

 are opposite in position, it is possible to obtain two leaves of the same age and 

 degree of maturity. One of them may be used for the control, the epidermis 

 being fixed at once, and the other allowed to wilt for any suitable period and 

 its epidermis fixed at the close. By taking a sufficient number of pairs of 

 leaves, observation may be taken so as to give a series, say of 1 minute or more 



*This method should prove useful in studying mechanical tensions in the leaf during 

 extended periods. 



