221 
1885.] Plants and the Structure of their Organs. 
species of one and the same genus. It appears that 
Asperula odorata, which inhabits moist and shady woods, 
possesses no means of protection against evaporation. On 
the other hand, species of the same group ( Asperula 
arvensis, tinctoria , cynanchica, galioides ) which all affeCt 
more or less sunny stations, are provided with more or less 
developed means of protection : thus the leaves are in some 
cases narrower, or their margins are spirally curled up, or 
there are special arrangements in the structure of the meso- 
phyll and the rind. Without doubt the stomata of all plants 
of most stations are more frequently open than those of 
plants which have to contend with want of water. 
The third method of investigation has been applied by 
Volkens to the flora of deserts, and especially of the Sahara. 
One of the groups there most generally occurring are the 
Leguminosse, especially the genera Genista, Spartium, and 
Retama. These plants are especially enabled to bear the 
dryness of the climate by their leaflessness. 
Our European species of Genista and Spartium proclaim, 
by their possession of leaves, that they are placed in more 
favourable circumstances as regards water. Spartium scopa- 
rium accommodates itself to the increased heat of the 
smallness of the leaves on its summer shoots, which are no 
longer triple, but single. Volkens is of opinion that all the 
dead and thick-walled elements within the vegetable body 
form in their totality a reservoir of water. 
