252 Wilson — The Mucilage- and other 
In the light of the above considerations there can be no 
doubt but that both sets of glands have the same origin. 
There is at the same time the greatest probability that the 
Mettenian glands are the primordial forms. 
With respect to the stalked glands on the calyx of 
Plumbago , one can hardly hesitate in affirming that they are 
extremely specialised forms of Mettenian glands seen in the 
typical condition on the sepals of Ceratostigma , Statics , etc. 
On the other hand, their homology with mucilage-glands 
may be regarded as substantiated, in the light of the fact that 
mucilage-glands are found on the outer and inner sides of the 
sepals of Asgialitis, and that the sessile mucilage-secreting 
glands of both sides of the sepals of Plumbago are identical. 
The contention is strengthened by the appearance on the 
inner face of the calyx of Vogelia of sessile glands homolo- 
gous with those of Plumbago. In regard to the elevation of 
the glands on pedicels it is well to recall that such happens 
occasionally in the axils of leaves of Statices, e,g. A. sareptana , 
and normally on the margins in Plumbago micrantha . 
Previous investigators have referred to the correlation which 
subsists between the climatic conditions under which certain 
species grow and the amount of calcium carbonate secreted by 
them. Volkens experimented with leaves of Limoniastrum mo- 
nopetalum cut from the plant. In the one case he removed the 
chalk-scales, and in the other left them over the glands. The 
result was that the former in an hour had lost by transpira- 
tion 26 per cent, of weight, and the latter 3 per cent.; and 
after three hours the one was quite withered, while the other 
remained fresh. From this he justly concludes that the 
purpose of the chalk-scales is to protect the plant from exces- 
sive transpiration, a necessary provision in its arid habitat. 
The plant in question affords an extreme and very intelligible 
case. From this example quite a series, involving all genera, 
could be constructed, showing gradual decrease in the number 
and size of the scales, until, as in almost all Armerias, no 
calcium carbonate appeared at all. This problem then sug- 
gests itself, — Were the Mettenian glands originally mucilage- 
