| Popis: |
The discussion here presented is based upon observations made during the past four years on the various species of Opuntia indigenous to Arizona. All of these plants, more especially the younger growth, are soft and fleshy, with large quantities of sap stored in the cortex and pith. No other plants so persistently retain their moisture when once secured. A thick epidermis with small sunken stomata and the evaporating surface brought down to a minimum, by the condensed form characteristic of the entire genus, enable them to remain green for months, even when continually exposed to the dry and scorching heat of our southwestern plains. They are alike at home on the plains and among the rocks of our mountains and foot-hills. Although of recent origin, compared with other large families of plants, they have lived for ages subjected to an environment which has placed them among the greatest economizers of water to be found in the entire vegetable kingdom. Not only are they economizers of water, but their tissues are, under ordinary circumstances, well provided with water, even after months of exposure to an average maximum temperature of i000 F. and untouched by dews or rains. Much of the tissue entering into their composition, consists of large thin-walled parenchyma cells, which serve as store-houses of water. These cells have the power of taking up water with great avidity after and during a rain, or when the ground is moist, and as a result the young branches become plump, smooth, thicker, and the tubercles less prominent. As the days become warmer and the rains less frequent these cells gradually give up their contained moisture and the joints become withered and wrinkled. This process is, however, very slow, as many species remain green a year or even longer, after every source of outside moisture has been withdrawn. The first of July, I 892, half of one of the flat joints of Opmntia basilaris was brought to my room and placed, without soil or moisture, in a small open box. The specimen was [356] |
