Page:Popular Science Monthly Volume 81.djvu/105

Rh species of opimtia (Opuntia vivipera) in which the roots of one and the same individual may be either fibrous or fleshy. Also species resembling Opuntia arbuscula, but possibly another species, which grows in the vicinity of Sacaton, Arizona, appears to have fibrous roots only. It has been found also that the seedlings of many cylindro-opuntias have fleshy roots. This last may be taken to be a temporary or juvenile stage, but probably is not, for reasons which will appear directly. With the above and other observations in mind, specimens of opuntias of several different species have been grown in saturated soils, with the uniform result that the roots formed in the saturated soil were fleshy. This result might be taken to indicate the immediate effect of an abundant water supply, but in the end it may be found that the result, in part at least, may be attributed to the air relation.

There is another relation which has not been referred to and which is of great importance, namely, the osmotic relation. This can be given briefly. A strong impetus to the study of this relation has recently been given by Fitting, who has shown that the shoots of certain desert plants may possess a very dense cell sap, so concentrated in fact that an osmotic pressure as great as 100 atmospheres has been determined, which pressure may even be exceeded. In the cells of the shoots of ordinary mesophytes the usual pressure is said to be from 5 to 11 atmospheres. While it has not been shown that the cell sap of the root hairs of such desert plants as have high osmotic pressures in the cells of the shoots is isosmotic with them, yet it has been assumed that the roots of these plants contain a very dense sap, as is probably the case. There is an apparently direct relation between the dryness of the habitat and the concentration of the plant juices, by reason of which the desert plant can absorb water from an intensely dry soil. As a rule the highest osmotic pressures, therefore, are to be found among perennials living in the driest situations, and during the most arid seasons. From this condition it is of interest to note that it is probably those plants in which the generalized type of root-system is to be found, or a type approaching this, that possess the most highly concentrated cell sap, since it is plants having this form of roots, as was noted above, which occupy the most arid habitats. We may conclude from this additional evidence that, so far as the Tucson desert is concerned, it is not the most deeply penetrating type of roots which are to be considered the desert form par excellence, but, quite the contrary, it is such a root as can both reach out widely and penetrate as deeply as the soil permits find in which there is developed a cell sap of extremely high concentration.