. A textbook of botany for colleges and universities ... Botany. Si2 ECOLOGY On the other hand, the passage of water and salts from the velameil through' the transfusion cells to the cortex is a slow osmotic process quite comparable to absorption by root hairs. The velamen is an organ of water accumulation as well as an organ of absorption, and it retains water for hours and even for days. Its significance is still more obvious when it is realized that only liquid water can be utilized by plants; orchids even decrease in weight in moist chambers unless watered. Hence it is not surprising that
. A textbook of botany for colleges and universities ... Botany. Si2 ECOLOGY On the other hand, the passage of water and salts from the velameil through' the transfusion cells to the cortex is a slow osmotic process quite comparable to absorption by root hairs. The velamen is an organ of water accumulation as well as an organ of absorption, and it retains water for hours and even for days. Its significance is still more obvious when it is realized that only liquid water can be utilized by plants; orchids even decrease in weight in moist chambers unless watered. Hence it is not surprising that epiphytes with ab- sorptive roots are confined to warm and humid climates, where rain or dew is almost continually available. Fig. 734. —An outline, as seen in cross section, of a dor- siventral orchid root {Aerantkus fasciola)jShowing the expanded upper portion (it), which con- tains most of the chlorophyll. —After Janczewski. The chlorophyU which is present in air roots probably is of some importance in food manufac- ture. Doubtless the presence of chlorophyll in these organs is related to light, since various soil roots develop chlorophyll when exposed to it. In Tae- niophyllum and in similar orchids with greatly reduced stems and leaves, the roots, which are the chief food-making rgans, are flattened rather than round, and lack the usual radial structure. The lighted side has a thick-walled exoder- mis and prominent cortical chlorophyll but little or no velamen, while the shaded side has a strong velamen, a thin- walled exodermis, and abundant root hairs (fig. 734). Absorptive air roots play only a small part in anchorage, though in some cases, especially in the flattened roots just cited, they adhere closely to the tree 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 Coulter, John Merle,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
