. Bulletin. Agriculture; Agriculture -- Arizona. Cold-Reststance in Spineless Cacti 125 lie, in an early stage, in a straight radial direction and take their ori- gin from the periblem, which direction may be observed here and there in Fig. 1. At a later age it is impossible to find out how the tissues were formed, as the cells of the green parenchyma sometimes divide longitudinally, and the thick-walled cells are so crowded that they no longer lie inradial Fig. 1.—Cross-section of a piece of joint of Opuntia castillae about two weeks old, showing regular character of cells. (Highly mag
. Bulletin. Agriculture; Agriculture -- Arizona. Cold-Reststance in Spineless Cacti 125 lie, in an early stage, in a straight radial direction and take their ori- gin from the periblem, which direction may be observed here and there in Fig. 1. At a later age it is impossible to find out how the tissues were formed, as the cells of the green parenchyma sometimes divide longitudinally, and the thick-walled cells are so crowded that they no longer lie inradial Fig. 1.—Cross-section of a piece of joint of Opuntia castillae about two weeks old, showing regular character of cells. (Highly magnified). It is hardly necessary to mention that the thick cuticle, the crystal-bearing layer, and its adherent thick-celled tissues, together with the slimy contents of the green parenchyma, are of great importance to the plant, as these protect the stem against a high transpiration rate, which would be disastrous for the species in a dry and hot climate. It would not be worth while to describe the thickness of the layers of that part which forms the integument of the stem (i. e., cuticle, epidermis, crystal-bearing layer, and thick- celled layer) if it were not observed that this integument is thickest in those species which are resistant to cold. More than this, physiological experiments demonstrated that the time of penetra- tion of a certain temperature varies greatly when integuments of different species of cacti are Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original University of Arizona. Agricultural Experiment Station. Tucson : Agricultural Experiment Station, University of Arizona
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