Notes . 
271 
Pleospora , Claihrospora , and Pyrenophora \ in which he describes this 
form living parasitically upon Sambucus nigra . 
The following quotation will show that Professor Berlese is also of 
opinion that this fungus does not cause any disease, but attacks only 
the most external parts and the dead tissue of the cortex ‘ La 
Pleospora herharum e un fungo saprofita, e che non determina alcuna 
malattia, e tutt’ al pm concorre a rendere pm pronte e piu efficace 
l’azione deleteria del morbo. lo lo riscontrai sui rami vivi di Sam - 
burns , ma e probabile che esso intacchi soltanta le parti piu esterne e 
di tessuto morto della corteccia/ 
A. E. SHIPLEY, Cambridge. 
OH THE MECHANISM OP THE STOMATA.— Until the 
publication of Leitgeb’s important researches on this subject, it had 
been generally assumed, from the observations of Moldenhauer 2 , 
Amici 3 , and von Mohl 4 , that the opening and closing of the stomata 
was a function of light ; light inducing opening, darkness closing. The 
explanation offered by von Mohl of the action of light is briefly 
this : that, on account of the chlorophyll-corpuscles which they 
contain, there are formed in the guard-cells, under the influence of 
light, osmotically active organic substances ; and that, under these 
circumstances, the guard-cells absorb water in such quantity as to 
induce a high degree of turgidity, and thus to enable them to over- 
come the compression exercised upon them by the adjacent epidermal 
cells. In darkness, these osmotically active organic substances are no 
longer formed ; consequently the high degree of turgidity of the guard- 
cells can no longer be maintained, and the stomata close. 
It was subsequently suggested by Sachs 6 that light may exercise 
a direct influence upon the molecular condition of the protoplasm of 
the guard-cells, of such a nature that, with increasing intensity of 
illumination, the protoplasm becomes more resistent to the escape of 
water from the cells ; in a word, that the opening of the stomata in 
the light is the result of the stimulation of the irritable protoplasm 
(Reizerscheinung). This suggestion, however, simply results in a 
1 Nuovo Giornale Bot. Ital. vol. xx, 1888. 
2 Moldenhauer, Beitrage zur Anatomic der Pflanzen, 1812. 
3 Amici, Ann. d. sci. nat., t. 2, 1824. 
4 von Mohl, Botanische Zeitung, 1856, p. 717. 
5 von Sachs, Vorlesungen iiber Pflanzenphysiologie, I st ® Aufl., 1882, p. 300 
(English edition, p. 250). 
