The anatomy of root contraction with special reference to Hyacinthus orientalis

The literature on the anatomy of root contraction is reviewed with special reference to recent work. This investigation, by light and electron microscopy, began with Hyacinth and was later extended to Gladiolus and Narcissus for the purpose of comparison with the findings of other workers. Contraction is the result of growth in the inner or middle cortex in which the cells extend radially and shorten axially, their variously orientated walls behaving differently. The horizontal walls are unusual in their structure. Even in uncontracted roots when potentially contractile, they resemble axial walls more than horizontal walls, in that the pit fields are orientated in relation to their potential for growth. In transverse sections of contracted roots, viewed by polarisation microscopy these walls appear as radially extended, highly birefringent thickened-plates with a crossed-fibrillar structure and with thin-areas , of very small birefringence, which become spaced further apart as radial growth and contraction proceed. Microfibrils in the thickened-plate fan out into the developing crossed-fibrillar framework of the radial walls. These walls extend radially and shorten axially. They also develop the marked crossed fibrillar structure already referred to, which results in the complete occlusion of their pit-fields. The middle lamellae between radial walls of adjacent cells break down, the older outer microfibrillar layers become torn and cohesion between adjacent cells is lost. The tangential walls show a passive wrinkling (ballooning) with points of adhesion in the region of pit-field areas. It is proposed that a mechanism of contraction based on these anatomical observations is brought about primarily by the radial extension of the horizontal walls and its concomitant effects on the axial walls. These observations and interpretations are discussed in relation to recent relevant work, in particular that of Sterling, who has proposed a different mechanism which seems to be quite incompatible with the anatomical facts.