i< ^^ ^ M 



RELATION BETWEEN " BLOOM AND DISTEIBVTION Oi' STOMATA. 99 



On the Eelation between the " Bloom " on Leaves and the Distri- 

 bution of the Stomata. By Francis Darwin, P.E.S., F.L.S. 



[Read 4th February, 1886.] 



Sachs* has pointed out that there is a connection between the 

 dititribution of stomata on leaves and their protection from wet by 

 the wax'like coating commonly known as " bloom." 



He says : — " It is especially the surfaces of leaves that are 

 well provided with stomata which seem to be protected against 

 the adherence of water. The leaves of water-plants such as 

 the Nymph^eaccfe, Polygonmn mnphibium, HydrocTiaris^ &c., are 

 thoroughly wetted on their lower surfaces, which have no 

 stomata ; but water runs off in round drops from the upper 

 surface, where the stomata occur. The meaning of this fact in 

 the economy of the plant is evident; the mouths of the stomata 

 would be closed by prolonged contact with water, and would 

 thus prevent the rapid ingress and egress of gas." t 



In the year 1878 my father was engaged in studying the 

 bloom on leaves, and it fell to my share to follow up the sug- 

 gestion of Sachs — that one function of bloom is to be found m 

 the protection of the stomata from wet. The mere fact that 

 stomata close when the leaf is wetted might lead us to expect 



that water interferes with their function, even if we had no 

 theoretical reasons for believing so. Barthelemy (as quoted in 

 the Botan. Centralblatt, vol. xix.) has recorded a fact demon- 

 etrating the closure of stomata by water. The leaves of JVi?- 

 lunibinm give out bubbles of gas when immersed in water and 

 exposed to sunshine, but the production of bubbles ceases if the 

 " bloom " is removed so that water comes into contact with the 

 stomata. The conclusion that the closure of the stomata is due 

 to contact with water mu^t, however, be cautiously received, for 

 it seems probable, as G-arreaut states, that the act of washing 



*Physiologie V^getale ' (French translation), 1868, p. 178. 



t In Lundstrom's interebting paper, ** Die Anpassungen der Pflanzen an 

 Eegen und Thau " (Nova Acta Reg. See. Sci. Upsal. 1884), the author points 

 out that in TrifoUum repens the under surface of the leaflets is wetted by rain, 

 while the upper bloom-protected surface remains dry. He connects this fact 

 with the occurrence of stx*)mata on the upper surface. 



t Ann. Sci. Nat. 1850. 



LINN. JOUKV. — BOTANT, \OXu XXII. 1 



