Page:Popular Science Monthly Volume 33.djvu/496

480 property of adaptation to desiccation so rarely appear among the higher plants?—why have the plants of the steppes and deserts, for example, to protect themselves against the perils of drought by so various anatomical features, of thick skins, corky bark, waxy and hairy envelopes, receptacles for water, etc., instead of simply drying up and reviving again in the rainy periods? The answer to the question is not hard to find. The maxim, "one thing is not suitable for all," is valid in the biological domain. That which works well in the little mosses is for various reasons not available to the larger phanerogams. First, the larger plants must continue to vegetate actively for longer periods, in order to prepare the amount of food required for the proper growth of their organs. Ever-recurring interruptions of their feeding by drying out would so retard the whole process of their growth, that in spite of their vital tenacity they would be at a great disadvantage in the struggle for existence. To this is added another not less weighty reason, that the mosses are so simple in their anatomical structure that the mechanical shrinking of the drying tissue involves no danger; the collaborated cells easily resume their original form and size on the accession of a new water supply. It is very different with the organs, far more complicated in their structure and composed of tissue of diversified kinds, of the more highly developed plants. In them extensive shrinkage would result in damaging tensions and distortions, and even cracks, for the limitation of which various mechanical protective adaptations would be required. Besides this, the mechanical structure of the tissues would have to have a proportionately enormous development, else the dry, brittle leaves and branches would be broken up by every gust of wind. A careful regard to the consequences of such an adaptation to complete desiccation should be sufficient to convince any one that it would be too dearly purchased. But in the case of many of the humbler plants insensibility to continuous desiccation is a life-question, and accordingly they have practically acquired that property. It is of equal interest from a physiological and a biological point of view that the protoplasm of the young individual should, by further development, gradually suffer the complete loss of so pregnant a property as that of reviving after it has been dried up.

The power of the mosses to endure repeated desiccation has recently been experimentally treated by G. Schröder, who obtained the interesting result that many of these plants can not only resist months of dryness without any harm, but also that they do not perish even under the strongest desiccation carried on in a drier with the aid of sulphuric acid. Plants of Barbula