Contributions to the Life History and Biology of Digitalis purpurea. I. The Development of the Ovule.

The young ovule of Digitalis purpurea has one integument four or five cells thick and a single-layered nucellus enclosing a spore mother cell from which a linear tetrad of megaspores is formed. The chalazal megaspore enlarges while three sets of nuclear divisions are occurring, crushing the three other megaspores and the nucellus so that in later stages of development an epithelium, derived from the inner epidermis of the integument, is the immediate investment of the embryo sac. Antipodal cells are delimited before the cells of the egg apparatus. Prom measurements made of the various parts of the ovule it is deduced that, until vacuolation starts in the primary embryo-sac mother cell, the integument compresses the parts within it, but that this vacuolation results in a counter pressure by which the three aborting megaspores, the nucellus and the inner part of the integument around the micropyle are crushed, the outer part of the last possibly being distorted so that the total length of the ovule becomes less. The possibility is discussed that the surface of contact between the integument and the nucellus delays the passage of solutes from the integument inwards, except where they form a continuous tissue at the chalaza, thus creating in this region a priority of supply which enables the chalazal megaspore to succeed and the antipodal cells to develop precociously. This research was carried out with apparatus purchased with a grant from the Dixon Fund.