Does intrusive growth of fusiform initials really contribute to circumferential growth of vascular cambium?

Abstract

Currently it is believed that intrusive growth of fusiform cambial initials adds to the circumference of the cambial cylinder: the initial cells multiplied by anticlinal divisions are produced in excess, and the excess cells are later eliminated from the cambial surface. The present study, dealing with the intrusive growth of fusiform initials in the cambium of Laburnum anagyroides Medik, suggests that addition of a radial file of initials owing to intrusive growth, or elimination of any such file, has no visible effect on the tangential dimensions of a given cambial sector, and that intrusive growth of fusiform initials and the elimination of excess initials occur in unison. The two events complement each other, and the gain in size of the growing fusiform cell is accompanied by a reduction in size of its neighbour cell, thus keeping the tangential dimensions of the cambium unchanged. Our findings on L. anagyroides find support from illustrations of previous studies, which we have re-examined and re-interpreted. Our data suggest that increase in the cambial circumference is largely due to the symplastic growth of the fusiform initials in tangential direction.