92 MacDougal. — The Water -balance of Desert Plants. 
brought into a shaded room from the open air. Still another specimen 
showed a distinct gain in the open during a night of drizzling rain. As 
these plants are provided with a dense armature of long curved spines (see 
PI. VII) which are hygroscopic,their dead tissues dryout under arid conditions 
and appear to take up water or aqueous vapour, as any bit of dry wood 
might do it. The bases of these spines are made up of heavy-walled 
tissues through which water might pass with difficulty. It is probable, 
therefore, that the changes in weight of Echinocactus due directly to 
humidity do not affect the succulence of the living tissues, and concern the 
dead spines only. 
The actual behaviour of the spines with regard to atmospheric moisture 
was tested in July, 1911. The spines from two ridges of a plant 50 cm. in 
height were detached and sent to the acclimatization laboratory at Carmel, 
California. The lot was placed upon a suitable sheet of paraffin paper and 
found to weigh exactly 20 g., in a sunny place, at noon on July 8. After 
exposure to the humid air in the open throughout the night, the weight was 
found to be 20-400 g. at 9.15 a.m. on the 9th, after some drying out had 
taken place. Desiccation occurred as before and the spines were again 
placed in the open and sheltered from precipitation, at 7 p.m. on the 10th. 
The night was foggy and misty and the spines absorbed so much that the 
weight had increased to 20-745 g. at 7 a.m. on the nth. The lot of spines 
was practically equivalent to those clothing Echinocactus No. 4, and was 
about one-tenth of the whole number on the plant from which they were 
taken. The total weight variation on such a plant due to imbibition and 
desiccation of the spines might, therefore, be about 7 or 8 g. daily, and on 
the largest individuals two or three times this amount. The proportion of 
weight variation due to such causes would be large in small plants. 
The conditions presented are different from those studied by Spalding, 1 
Schonland, 2 and Marloth. Marloth 3 detected positive absorption of water 
and water vapour by the hairs of Mesembryanthemum and Crassula , and 
the structure of one type of trichome in the younger stage is such as to 
permit the passage of water into the living cells. Lloyd found that water 
absorbed by the loose bark of Fouquieria passed into the living tissues, and 
that the consequent increase of the water-balance was followed by the 
unfolding of new leaves in stems, quiescent at the beginning of the experi- 
ment. 4 Miss Delf demonstrated that water in liquid form as well as a vapour 
might pass the epidermis of the green parts of Suaeda maritima and Salt- 
1 Absorption of Atmospheric Moisture by Desert Shrubs. Bull. Torr. Bot. Club, vol. xxvi. 1906, 
P- 367 * 
2 On the Absorption of Water by the Aerial Organs of some Succulents. Trans. Roy. Soc. South 
Africa, vol. i, part II, 1910, p. 395. 
3 Notes on the Absorption of Water by Aerial Organs of Plants. Trans. Roy. Soc. South Africa, 
vol. i, part II, 1910, p. 28. 
4 The Artificial Induction of Leaf-formation in the Ocotillo. Torreya, vol. v, 1905, p. 175. 
