. The biology of flowering plants. Phanerogams; Plant physiology; Plant ecology; Plant Physiology. 288 THE BIOLOGY OF FLOWERING PLANTS another mechanical function fulfilled by roots and hypo- cotyls. In rosette plants with a vertical root stock such as the primrose, or in plants like the crocus, in which new corms are formed yearly at a higher level than the old, the new growth would soon project above the ground if it were not pulled down. This is done by special " con- tractile " roots (Fig. 42). They are usually rather thick, and, after becoming firmly fixed, they undergo an activ
. The biology of flowering plants. Phanerogams; Plant physiology; Plant ecology; Plant Physiology. 288 THE BIOLOGY OF FLOWERING PLANTS another mechanical function fulfilled by roots and hypo- cotyls. In rosette plants with a vertical root stock such as the primrose, or in plants like the crocus, in which new corms are formed yearly at a higher level than the old, the new growth would soon project above the ground if it were not pulled down. This is done by special " con- tractile " roots (Fig. 42). They are usually rather thick, and, after becoming firmly fixed, they undergo an active contraction by the alteration in the shape of definite tissues. The epiderm does not contract, and comes to have a curious wrinkled appear- ance. This is well seen in the roots of young crocus corms in early summer. In some seed- lings, too, a contraction of the primary root and of the hypocotyl brings the little plant well into the soil; this is particularly the case in plants in which the hypocotyl takes part in the formation of storage organs, which are thus pulled down to a definite Fig. 42.—Contractile roots of Chlorcea membrajiacea: the ground level is indicated by the dotted line ; the base of the first root has been pulled far down by the action of the second. Nat. size. (After Rimbach.) § 3. Mechanical Features of THE Stem The requirements of the stem are very different. Did the weight of the foHage and branches bear downwards quite symmetrically the stem would be subjected to longi- tudinal compression like a pillar. The stem is, however, normally subject to continuous movement by the wind, and, even in still weather, the bearing of the crown is never exactly central. The result is that the stem is principally. 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 Skene, Macgregor. New York,
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