It is very difficult to understand how the sap or water rises in the trunks of tall trees; we know that along the path of the sap inside, the root-hairs and other cells in the root, the various cells in the stem, and finally those of the leaf, are all kept supplied and distended or swollen out with water. All these living cells seem to have the power of absorbing or sucking in water,[3] and eventually they are so full and distended within, that the internal pressure becomes almost incredible. Wieler found in the young wood of a Scotch fir that the pressure was sixteen atmospheres, or 240 lb. to the square inch. Dixon, when experimenting with leaf-cells, found ten, twenty, or even thirty atmospheres (150 to 450 lb. to the square inch). No locomotive engine has cylinders strong enough to resist such internal pressures as these. It is an extraordinary fact, and one almost incredible, that the cells can stand such pressures. [3] The ascent is assisted by the osmotic absorption of water at the root and by evaporation at the leaves. [Illustration: _Stereo Copyright, Underwood & Underwood_ _London and New York_ A SENTINEL PALM IN THE ANDREAS CAÑON, CALIFORNIA This and such palms are often placed at the mouths of cañons to indicate water, and may, indeed, thus save the lives of passing travellers.] Yet these minute living cells not only exist but work at this high tension, and, in some cases, they live to about fifty years. In this favoured country of Great Britain, it is unusual to find any serious lack of water. But in Italy or Greece, every drop of it is valuable and carefully husbanded. Sometimes in such arid dry countries, a small spring of water will form around itself a refreshing oasis of greenery surrounded everywhere by dreary thorn-scrub or monotonous sand. All the plants in such a spot have their own special work to do: the graceful trees which shade the spring, the green mosses on the stones, the fresh grass and bright flowers or waving reeds, are all associated in a common work. They protect and shelter each other; their dead leaves are used to form soil; their roots explore and break up the ground. It is true that they are competing with one another for water and for light, but they are all forming a mutual protection, and producing an annual harvest. In a climate like our own we cannot, like the Greek, suppose a Nymph in the shape of a lovely young woman watching over the spring, for she would infallibly suffer from rheumatism and ague. But every living cell in every plant in such an oasis depends upon the water of the spring. All the plants there form an association which can be quite well compared to a city or some other association of human beings. They do compete, for they struggle to do the most work for the good of the community, and they incidentally obtain their livelihood in the process.