198 PROCEEDINGS OF SOCIETIES. 



creation that are spread around us in such boundless profusion, I 

 propose to direct attention to some points of interest that may have 

 escaped notice, even of those well acquainted with the general habit 

 and appearance of a plant that is met with in so many localities. 



The Parietaria officinalis belongs to the Urticaceae or nettle 

 family, which, although abundant in tropical regions, is represented in 

 England by very few species ; the stinging-nettles, the hop, and the 

 elm, being the only other members of the order. I do not, however, 

 propose to enter into any detail of the characteristic or common 

 peculiarities of these genera, but merely to point out some points of 

 interest in the leaves of the common pellitory. 



The only description generally given of these leaves in botanical 

 works is, that they are slightly rough or hairy, and Loudon in his 

 Encyclopaedia notices that they are marked with pellucid dots. If a 

 leaf is placed in water under the microscope, or, better still, if a small 

 section is made and examined in the same way, the hairs are very 

 plainly seen, and are of two kinds. The most abundant consists of 

 long slightly curved transparent spine-like hairs, with rather blunt 

 points, apparently hollow at the other extremity, and attached to the 

 centre of some cells arranged somewhat in a stellate manner, and 

 larger than those forming the general substance of the leaf. Inter- 

 spersed with these, but not so abundant, are found small recurved 

 hairs, about one-fifth the length of the others, which in shape and 

 peculiar curve exactly resemble small fish-hooks ; these are scattered 

 apparently at intervals, especially on the younger leaves, but are less 

 abundant on the older leaves towards the base of the stem. But the 

 structure of most peculiar interest in these leaves consists in the so- 

 called " pellucid dots " of Loudon, which may be readily seen by 

 holding a leaf up to the light. If the leaf is placed in water, and the 

 upper surface examined with a half or quarter inch objective, these 

 dots are seen to consist of seven or eight rather large cells, radiating 

 from the sides of a centre cell, which appears slightly raised above 

 the surface of the leaf, so that the surrounding cells appear to slope 

 from it to the surface of the leaf ; below these, and in the parenchyma, 

 or substance of the leaf itself, is a large single cell, within which 

 is suspended a sub-globular or slightly pear-shaped mass with a papil- 

 lated surface, but with no clearly defined crystalline structure. These 

 bodies are known as Sphaeraphides, and have also been called " Cry- 

 stoliths " by Continental writers ; they are sufficiently large and hard 

 to be easily separated from the parenchyma of the leaf when thin 

 sections are made, or small portions torn up under the microscope. 

 When treated with muriatic acid they dissolve rapidly with consider- 

 able ebullition, and when burnt are reduced to a white powder ; there 

 can be no doubt that they are, therefore, chiefly composed of lime, 

 and probably in the form of carbonate. They differ from the true 

 Eaphides, so abundant in many plants, by being almost amorphous, 

 though occasionally a slight semi-crystalline appearance may be 

 detected in small fragments if examined with a quarter objective. 

 Although not so often noticed as true Raphides, they are character- 

 istic of many tribes of British plants — as the Caryophyllaceae, Gera- 



