Natural History. 391 



which he saw circulating in the tail, as the same as those which other 

 observers have remarked as a little cloud, when a globe of pollen has 

 been broken. 



The pollen of the flower of the cucurbit a pepo, are globes, which 

 when moistened presented, at different points of their surface, very- 

 transparent vesicles, at the summit of which were adapted small 

 opaque covers with a projecting spine in the middle; this cover ap- 

 pears to act as a valve whilst the vesicle is within the globules. if 

 the pollen be dipped in alcohol, before being placed in water 

 they do not break, and the phenomena of the vesicles are better 

 observed. 



The pollen of the cichorium inhjbus'is of a regular dodeca'edral 

 form, with pentagonal faces ; put on to water it bursts at one of the 

 faces and throws a liquid to a distance twice its own diameter, some 

 of the other faces swell and produce vesicles, analogous to those 

 before mentioned, but without the operculum. 



On the Epidermis. The epidermis of the leaves of a great number 

 of plants examined by M. Amici, is a tissue formed of a layer of 

 cells, independent of those of the parenchyma which are covered by 

 it. It is white, transparent, and may be removed without laceration 

 of the subjacent parenchymatous layers, of which each has its par- 

 ticular membrane, which adheres only by contact to the epidermis. 



M. Amici refutes the opinion of those who affirm the common 

 nature of these two substances, by pointing out that in many cases 

 (diant/ius caryophyllus for one,) the cells of the epidermis are quadri- 

 lateral, whilst those of the parenchyma are cylindrical tubes, of vari- 

 ous lengths, perpendicular to the plan of the epidermis. But these 

 vary in different plants, and are sometimes very singular. The dif- 

 ference in the figures of these cells may readily be seen without re- 

 moving the epidermis, by only changing the focus of the microscope 

 by a quantity equal to the thickness of the epidermis ; they are thus 

 presented alternately to the eye, and their want of correspondence 

 made evident. The spaces which the varied dispositions of the pa- 

 renchyma produces are filled with air ; and they correspond with 

 areas of an oval form in the epidermis, in the centre of which may 

 be observed apertures, sometimes open and sometimes closed. In 

 leaves of the ranunculus rcpens and ruta graveolens, the organ ter- 

 minated by these orifices is a small bag or purse, which is opened or 

 closed by a sphincter according to circumstances, not mercty spon- 

 taneously in the living plant, but at the will of the observer. They 

 are generally open in sun light, closed in darkness ; large when the 

 leaf is dry, narrow when it is moistened. In the ruta graveolens, 

 when the pores are open, the parenchyma composed of small green 

 tubes may be seen, when closed the green disappears, and the ori- 

 fices take an ash colour. 



With regard to the functions of these pores, it is concluded, that 

 tkey are not intended for absorption of water, because they close 



