412 WALTER GARDINER. 
means of the one or more water-pores, the development and 
structure of which we have previously studied. 
Mode of action of the gland.—Since the aim of this paper 
is to regard the gland solely from a histological standpoint, 
the mode of action will be only glanced at in a very cursory 
manner. In the daytime, when the temperature is high and 
transpiration very vigorous, the secretion of drops of water 
does not take place, and even if water be secreted by the 
gland cells it is evaporated as fast as formed. But at 
night, when the temperature falls, causing decrease of tran-- 
spiration, the walls of the vessels and even the vessels 
themselves become gorged with water, and every facility for 
secretion by the gland is offered. ‘Then drops exude 
through the water-pores, subsequently filling the pit. Now 
the portion of the lobe which intervenes between the gland 
and the edge of the leaf slopes downwards and outwards, 
and it is on this side especially on which the hairs sur- 
rounding the pit are most abundant. As a consequence, 
when an excess of water is secreted and the pit overflows, 
the water will tend to collect on the margin of each lobe. 
The water, charged as it is with carbonic acid, holds in 
solution a quantity of calcic carbonate. As the water eva- 
porates and the carbonic acid gas comes off, the chalk is 
precipitated, and that which is deposited in the vicinity of 
the pit, tends to aggregate around the hairs and becomes 
thus firmly held and prevented from falling into the pit 
and stopping up the water-pores. In spite of this special 
provision, however, the older glands frequently become 
inefficient on account of being choked, the pit becoming 
completely filled with the very large concretions «f chalk 
formed. 
The whole phenomenon may be easily seen by placing a 
bell jar over a vigorous plant. Since the air becomes 
saturated with aqueous vapour, transpiration is reduced and 
large drops of water are secreted. On removing the jar, 
the water rapidly evaporates and a deposit of chalk is 
formed. 
Glands in general.—As far as I have had an opportunity 
of examining water-glands, it appears that the gland of 
Saxifraga crustata is the most highly differentiated of all as 
regards the special provision made for the deposition of the 
calcic carbonate, the distinct differentiation of the gland 
tissue, the well-marked endodermis, the extreme granularity 
of the protoplasm, and the activity of function. 
Next come the glands of the Crassulas, where there are 
