FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 325 



another, so that taken together they form a layer which spreads over the other 

 epidermal cells like a coat of mail. The ordinary epidermal cells are small and only 

 slightly thickened on their outer walls, as shown in the illustration above. The 

 cells which are placed together to form the armour, however, are enlarged in quite 

 an unusual way; their stalk-like base, looking as if wedged in between the ordinary 

 epidermal cells, is indeed comparatively large, but the bladder-like swollen portion 

 exhibits dimensions which are about six hundred times greater than those of the 

 ordinary epidermal cells. The vesicles are closely packed together, and become 

 almost cubical by the mutual pressure they exert on each other. Where a space 

 might occur, the bladders form protuberances and bulgings at the side which fit in 

 together in such a way that a completely closed coat of mail is the result. The 

 expression " coat of mail " is the more justified here since the swollen bladder-like 

 cells of Rochea are as hard as pebbles. Large quantities of silica are present in the 

 cell-walls, and by burning them a complete skeleton in silica can be obtained, as in 

 the case of the silica-coated Diatomacese. It needs no further explanation that in 

 the dry season such a coat of annour affords to the succulent cells it covers an 

 excellent protection against evaporation. 



There is, however, still another point to be considered. The vesicular swollen 

 cells on fully-grown leaves are still occupied by protoplasm, which forms a thin 

 layer round the walls, while in the centre is a large cavity filled with cell-sap; it is 

 only in older leaves that the bladder-like cells become filled with air. As long as 

 they contain watery cell-sap they serve as reservoirs of water from which the green 

 chlorophyll-bearing cells below can obtain supplies at the periods of greatest 

 drought, when all other sources are exhausted. This fact, that the water-reservoirs 

 are situated on the exterior of the plants, where there exist so many aids to exhala- 

 tion, shows how well the silicified walls of these bladders function. They may be 

 compared to glass vessels whose mouths are directed towards the green tissue, and 

 whose walls allow absolutely no water to pass through. 



FORM AND POSITION OF THE TBANSPIBING LEAVES AND BRANCHES. 



The enlargement of the green leaf -surface has been already explained as a means 

 of increasing transpiration, which is of special importance when the plants con- 

 sidered grow in damp air. Similarly a diminution of the green surface signifies a 

 restriction of transpiration. This relation is illustrated by the fact that in all floral 

 areas, in which the activity of the vegetation is restricted or entirely stopped by 

 increasing dryness, the foliage of the plants is not so widely outspread, i.e. it under- 

 goes a diminution. It is also a well-known fact that one and the same species, 

 if grown in a dry sunny position, will exhibit smaller, and in particular, narrower 

 leaves than when it has been grown in a damp situation. This is well seen in 

 passing from the mountainous districts bordering the Hungarian lowlands to the 

 plains of the lower regions. A number of shrubs and herbs, Anchusa officinalis, 

 Linum hirsutum, Alysswm montanuTn, Thymus Marschallianus, &c., exhibit on 



