MECHANISM OF ROOT-CONTRACTION IN BRODIAEA LACTEA

In a recent paper in the American Journal of Botany, F. H. SMITH (7) has given an interesting account of the contractile roots of Brodiaea lactea. These show features that clearly differentiate them from the ordinary monocotyledonous types of contractile root which the work of RIMBACH (4, 5, 6) has made familiar. In the latter types the stele remains straight, owing to the fact that the taut contractile tissue lies just outside it. The cells of this active tissue, the inner cortex, grow wider and shorter, as described by DE VRIES for other types (13). Room for their transverse growth is obtained in many cases by progressive collapse of the outer cortical cells. In Brodiaea lactea the outer cortex does not collapse in this way and in the later stages of contraction the stele becomes greatly distorted, which shows that the tissue immediately surrounding it cannot be actively contracting and so longitudinally taut. According to SMITH'S account it seems that there may be two phases of contraction, the first, during which the stele remains straight, brought about in the ordinary way by growth contraction of the cortex while the root is still swelling; the second, after swelling is complete, involving the distortion of the stele and progressive collapse of transverse zones of the cortex. The alternation of turgid with collapsing transverse zones parallels in so remarkable a way the phenomena described by the writer for the Dicotyledon, Oxalis incarnata (9), that it seems worth while to draw attention to the similarity and to suggest that the explanation offered of the mechanism of the one may be applicable to the other. SMITH'S explanation of the later phase of contraction is not convincing. He appears to attribute it to the distortion of the stele in consequence of the unequal distribution around it of turgid growing cells exerting on it unequal lateral pulls. The turgid layers, owing to the loss of turgor of layers above and below, are released from vertical pressure, round off and so diminish their transverse dimensions and pull on the stele. If this were the explanation the stele would be under longitudinal tension. Transverse components of this tension would be borne by the turgid tissue pulling on the convexities of the stele. If, however, the stele is compressed during the first phase of contraction, it is less likely to offer resistance to extension. Besides, a glance at the violence of its distortion makes it impossible to regard it as a taut cord. SMITH refers to the tearing of the tissue just out721