THE MORPHOLOGY AND EMBRYOGENY OF SEQUOIA GIGANTEA

WHEN ONE considers the difficulties encountered in collecting material for a study of Sequoia gigantea, he comes to appreciate one of the reasons why this species has been neglected by morphologists. In cultivation at lower altitudes the ovules and seeds of the cones that may occasionally be found are usually abortive. In the mountainous region of the Sierras where this species is native and reproductive, the trees are so large that one cannot hope to obtain cones from them without expensive equipment and preparation. The fact that the cones persist on the trees for many years after the seeds are mature (Buchholz, 1937, 1938a) has not been generally understood and adds to the difficulties confronting the collector. To be sure, squirrels often cut off cones which may be picked up under the trees, but these are usually the older ones. Various individuals have collected and sent me cones at different times during the past twenty years. All cones received were found to be fully matured. During the spring and summer of 1936 the writer went to California for the purpose of making a study of the Sequoias. The only practical plan worked out at that time which would not entail expensive equipment was to use the occasional cones produced on the small second-growth trees. Trees less than 75 years old are abundant in Whitaker Forest, a preserve owned by the University of California near Redwood Canyon. They are also abundant in the cut-over region near General Grant National Park. During late June and early July while the new cones are still only partially grown, they may be recognized with field glasses by their smaller size and lighter color. It was found (Buchholz, 1938a) that while only one tree in several hundred in this second-growth stand may bear one or two cones (always near its top), they may be marked and mapped for collections during the summer when the cones are in the desired stage. About eight such trees were found, and the cones collected from them furnished me with the material for a study of the embryogeny. The stage of fertilization was not included. The cone collected nearest that date happened to be a teratologically misshapen specimen containing very few normal ovules. Some of the other collections were spaced a little too far apart in time, but the .cones obtained were excellent and yielded an abundance of embryological material in their respective stages both before and after fertilization. It was very fortunate and timely for my work that Looby and Doyle (1937) recently published their account of fertilization and the proembryo from material obtained in cultivation in Ireland. Had the writer planned a cooperative research with these investigators so as to pool all efforts in a study of the embryogeny of the big tree, a more 1 Received for publication December 12, 1938. precise division of labor could not have been outlined. Their account has supplied the stages missed by the author and enables him to describe the embryogeny with a satisfactory understanding of the proembryonic stages. It is of great importance to know that the proembryo of Sequoia gigantea has a free nuclear stage and to know how this proembryo is organized. The younger stages observed by me and shown in figures 23-25 represent the stage in which Looby and Doyle left off in their account. Thus, it is now possible to give a fairly complete general outline of the early embryogeny of this species. The ovules were collected and killed in Nawaschin's solution and in formalin-acetic-alcohol. The latter killing solution was found more satisfactory on the whole. Some of the material was cut serially in paraffin and stained by the usual methods. Nearly all the material that was ultimately found useful in a study of the later embryo was dissected material, the preparation of which has been described in considerable detail elsewhere (Buchholz, 1938b). For the last stages before fertilization and all subsequent stages, the seed coats must be removed; otherwise the seeds cannot be cut, because there is hardened tannin jell in the cells. The nucellus is thin and may be cut in paraffin without difficulty. Sections were cut in longitudinal axis and transversely. The most satisfactory orientations for cutting were those giving transverse serial sections of the nucellus. The literature on the morphology of Sequoia gigantea is very meager. Strasburger (1872, 1879) described the cones of S. sempervirens, confining himself very largely to external morphological features and the vascular supply of the cone scale. Arnoldi's account (1900) is the only earlier one which gives anything specific on the internal morphology of the Big Tree. His studies were made from material found in cultivation in the old world and do not include the embryo. The accounts of Shaw (1896) and Lawson (1904) were confined to S. sempervirens. Looby and Doyle's investigation covering fertilization and the proembryo is the first and only investigation of the Big Tree which includes a detailed account of these stages. In the following account of the gametophytes I am adding some new observations and repeating here and there a number of features covered by previous investigators. This duplication appears necessary in using my figures in the presentation. Figure 1 is a drawing showing the upper surface of a cone scale just before pollination. At this time the ovules are still very small and immature, consisting only of a nucellus bordered by the integument, which is merely a narrow rim. They may be described as appearing erect at this stage. With the enlargement of the cone scale after pollination, the ovules grow rapidly and soon develop to the