Female cone development in Thuja occidentalis

The development of female cone in Thuja occidentalis was investigated by means of SEM together with conventional sectioning technique. It was observed that the female cone initiation in T. occidentalis occurred in August by the transition of vegetative apices into seed cone apices in Beijing. Female cone in T. occidentalis consisted of 4～6 pairs of decussate bracts. Usually, two or three pairs of the fertile bracts in the middle of the cone subtended eight to twelve ovules. The first structure observed in the axil of the fertile bract was a broad swelling, on which two ovule primordia initiated. Furthermore, each ovule primordium differentiated a collar like integument around the base of the nucellus, and the integument enclosed nucellus progressively and overgrew the nucellus, forming an ovule with a micropyle. During the ovule initiation, it was further noticed that three ovule primordia were initiated in each of the axil of lower fertile bracts in half of the female cones observed. However, only two ovules developed, for the middle primordium aborted in later stages, which indicated that the female cone of T. occidentalis might be derived from that with more ovules per bract of the ancestor by reduction of the number of ovule per bract. We also preliminarily investigated the vascular system development of the cone. There was only a middle trace in the bract of young cones. After pollination, with the intercalary growth of the bracts, 4 to 8 vascular bundles developed in the bracts. However, the new formed vascular bundles did not show inverted orientation, which was reported as a feature in the cone development of Cupressaceae.