Watson.—Sphagna: Habitats, Adaptations , Associates . 545 
a character which is fairly constant for a particular species under similar 
conditions. Owing to this method the plant is able to obtain its mineral 
food from very dilute solutions, and the author has long held the view that 
it can only grow well after fresh water has been supplied. 1 It may be 
submerged in water that is useless for the due discharge of its physiological 
functions since it is too acid in character and not available owing to its high 
osmotic activity; it is only when the solution is considerably weakened (as 
after rain) that the plant resumes its normal growth, the death and decay 
of the cells in the lower portions of the plant being largely due to the 
inability of the cells to use the water after it has become strong in ‘ humic * 
substances by concentration during a period of dryness. 
Sphagnum seldom occurs in standing pools and stagnation of the water 
eventually kills it. It may occur in deep bogs, but the cells which are 
normally active are those towards the surface, and therefore in a position to 
profit more directly by the influx of dilute solutions. The normal habitat 
of many species, e. g. .S. cymbifolium , S. papillosum, S. acutifolium , is on the 
margins of bogs, or in other places where an influx of fresh water is readily 
perceived by them. Sphagnum is a plant which has rapid growth in suit¬ 
able conditions, and if very dilute solutions supply the mineral and nitro¬ 
genous substances necessary for metabolic processes, a large amount of these 
solutions will be required, and it is possible that 
(1) the solution is absorbed by the plant and the redundant water got rid 
of [a) by excessive transpiration from the chlorophyllous cells, or 
(< b ) by evaporation from the hyaline cells ; 
(2) there is a special method whereby a colloidal substance present in the 
cell-wall adsorbs the base of the mineral salt. 
When the plant relies mainly on the latter method the surrounding 
water becomes acid and may so interfere with endosmosis that sufficient 
water for physiological purposes is not available, and so devices protective 
against loss of water may occur. The structure of the plant is such that 
available water is quickly taken in by the hyaline cells, and passage of 
water up or down the surface of the plant is facilitated. ^For example, in 
.S. cymbifolium , the hyaline cells with pores on their outer surfaces will 
quickly absorb the water during of after rain, whilst the pendent branches 
preserve a watery communication for diffusion of the mineral salts to the 
cells and of the liberated acids from the cells. 
This special method does not exclude the more normal methods of 
obtaining mineral substances, is possessed by all Sphagna to some extent, 
and in most species appears to be the chief method. We can roughly place 
the species in three groups : 
1 Goebel, in Organography of Plants, has a note on the habitat of Sphagnum. . He says, 
1 Sphagna chiefly live upon rain-water; they grow in places where the water only contains a small 
amount of mineral substances necessary for their nourishment, so that a profuse water-evaporation 
is necessary 
