2o6 THE BIOLOGY OF FLOWERING PLANTS 



delayed. The dew might also block the stoma and even 

 infiltrate the intercellular spaces of the leaf. 



Recently Erban (191 6) has investigated the distribution 

 of stomata on leaves which show nyctinastic movements. 

 She finds that in those cases where the leaflets fold down- 

 wards, so that the lower surfaces come together and are 

 " protected," the stomata are wholly, or almost wholly, 

 confined to the lower surfaces. Twenty species of Oxali- 

 daceae, e.g. Oxalis hedysaroides and Biophytum sensitivum, and 

 Leguminosoe, e.g. Rohinia pseudacacia, Cytisiis Laburnum^ 

 all show this arrangement, and of these fourteen had no 

 stomata on the upper surface. The result is the same 

 whether the leaf is pinnate or trifoliate. Where the leaflets 

 fold upwards, so that the upper surfaces come in contact, 

 the stomata are most numerous on the upper surface, again 

 the " protected " surface. Seven species were examined, 

 and, though none had stomata confined to the upper surface, 

 the distribution is quite unmistakable ; on an average the 

 upper surface bore twice as many stomata as the lower. 

 In a third class of plants both sides of the leaflets are partially 

 " protected " in the night position, owing to partial covering 

 of one leaflet by another. This is seen, for instance, in the 

 clover, where the two side leaflets fold together, and the end 

 leaflet rises so as partly to cover them ; in the sensitive 

 plant a twist in the pulvini brings each pair of leaflets 

 forward, as well as up, so that they overlap the basal parts 

 of the pair in front. The conditions here are more complex, 

 as is also the stomatal distribution. In Mimosa the stomata 

 are most numerous on the upper surfaces, and on the lower 

 halves of the under surfaces, while at the completely un- 

 covered tips of the under surfaces they are few or absent. 

 In some other plants examined no definite relation between 

 protection and stomatal number was found. The most 

 interesting case, that of Mars ilia quadrifolia, though it is a 

 fern, may be described. The four leaflets fold together so 

 that two lie completely between the other two. On the 

 two inner leaflets, with both sides " protected," the stomata 

 are equal in number on the two surfaces The two outer 



