8 
F. Cavers. 
through in plants growing out of water. In a few species ( Sphagna 
cymbifolia) the cells of the cortex resemble the hyaline leaf-cells in 
having strengthening fibres and pores on their walls (Fig. 61), but 
in most species these cortex cells are devoid of fibres and pores. 
The tissue within the cortex is differentiated into an outer zone of 
narrow thick-walled cells and a central region in which the cells are 
wider and shorter and have thinner walls. In the outer zone the 
long narrow cells have pits on the walls, but later the walls become 
uniformly thickened and the pits are obliterated. In the branches, 
of both the outstanding and the drooping types, some of the cells of 
the single-layered rind grow large and have their outer wall per¬ 
forated at the upper end by a circular or oval hole, and as this end 
often projects outwards these cells resemble a bottle with a curved 
neck; these “retort-cells,” which are especially conspicuous in 5. 
molluscum (Fig. 62a), stand at one end of the insertion of a leaf, and 
they do not occur in the Cymbifolium-section, in which the cortex 
of both the main stems and the branches consists of perforated cells 
with fibrous thickenings. 
Many inaccurate statements are found in text-books concerning 
the biology of Sphagnum. Oltmanns (24) was the first to make 
careful experiments on the remarkable capacity of Sphagnum for 
absorbing and giving off large quantities of water. In most species, 
the water is drawn up by capillary action, an extremely effective 
system of capillary spaces being formed by the drooping branches 
which hang down against the stem and overlap each other. If the 
tufts of branches be removed from the end of the stem, which is 
then dipped into water, the plant remains dry, hence the stem-tissue 
does not serve a conducting function. But in S. cymbi folium and 
its allies, in which the cortex cells have pores and fibres on the 
walls, the removal of the pendent branches does not stop the 
drawing-up of water, though even here water passes up much more 
rapidly when these branches are left intact; in these species no 
water ascends at all when both the pendent branches and the 
cortex are stripped from the end of the stem. As is well known, 
Sphagnum rarely grows on limestone soils. According to Paul (26, 
27), calcium carbonate is very injurious to Sphagnum, even in small 
quantities; he grew various species in water containing lime and 
found that the plants soon perished. He states that not all salts of 
lime have an equally injurious effect, and that alkaline salts of 
potassium and sodium are as deleterious as those of lime. Various 
other writers have stated that the peat-mosses give an acid 
