154 Miscellaneous. 



of the island, filling the hollows in the plains, and sometimes, in the 

 course of a few hours, obliterating all traces of pathways, and thus 

 bewildering the newly-arrived traveller. I was in the leeward side of 

 the island when the first sand-shower occurred, and the residents 

 differed in opinion as to its source, some saying that it came from 

 the beach and sand-hills on the windward side of the island ; while 

 others, more correctly, as I consider, attributed its origin to the 

 African Desert. I had soon an opportunity of ascertaining that they 

 did not originate on the island itself, for I witnessed a sand- shower 

 of considerable density over the sea to windward of the island^ be- 

 tween which and the African coast no land intervened ; and I there- 

 fore came to the conclusion that that coast was its source." 



4. "Analogy between the serial arrangements of the Leaves of 

 Plants and Crystalline Forms," by Mr. WiUiam Mitchell. 



MISCELLANEOUS. 



On the Causes of the Opening and Closing of Stomates. 

 By Hugo von Mohl. 



In this memoir Von Mohl corroborates by actual experiments the 

 general impression, the truth of which had not been demonstrated, 

 that stomates shut when the guardian-cells collapse, and open when 

 they become turgid. 



The opening of the stomate is guarded by two crescent-shaped 

 cells, the guardian-cells, which generally take the following form. 

 On their external surface each bears a cuticular projection, which is 

 usually formed by a thin membrane ; in other cases, however, it con- 

 sists of the cell-wall considerably thickened, or the cell- wall is some- 

 times even thick enough to form a salient protuberance. The edges 

 of these projections unite at both ends of the stomate, so as to 

 make an orifice above the true opening of the stomate ; this orifice 

 may be wider or narrower than the true opening. It leads into a 

 continuation of the true opening, filled with air, and lying above the 

 opening ; this Von Mohl calls the anterior cavity, or antechamber 

 {Vorhof)y and the opening, the orifice of the antechamber. It is 

 bounded on both of its lower sides by the upper part of the lateral 

 surfaces of the guardian- cells, these surfaces being concave horizon- 

 tally and convex vertically. Turned towards the stomatic cavity, on 

 the lower side of the guardian-cells, there lies in most plants another 

 projection like that on the upper side, but generally smaller, by 

 which a posterior cavity, corresponding to the anterior cavity, is 

 separated from the cavity of the stomate. 



A transverse section usually shows that the thickness of the walls 

 of the guardian-cells is very unequal in different places ; the part of 

 the wall contiguous to the epidermal cells is generally rather thin, 

 so that these cells must prevent the guardian-cells from swelling out 

 at this pa)"' 



