北美香柏雌球果的发育

摘要

用扫描电镜(SEM)观察了北美香柏Thujaoccidentalis雌球果的发育过程。在北京 ，北美香柏的雌球果是在八月初由营养芽转变而来，雌球果一般有4～6对苞片，中间2～3对可育，每一苞片腋部着生两枚胚珠。在可育苞片腋部最先观察到一扁平的隆起，并在其上分化出两个胚珠原基，接着分化出珠被和珠心，最后形成扁平而两侧对称的胚珠。在北美香柏雌球果发育过程中，约一半的雌球果在2～3对可育苞片中位于下面的1～2对的腋部产生3个胚珠原基，中间一个较小，并在以后的发育中逐渐退化。由此推测北美香柏的雌球果可能是由祖先类群中每一苞片具多于2个胚珠的雌球果演化而来。在光镜下对雌球果维管系统的观察发现，传粉前幼小雌球果的苞片内仅有一束维管束，传粉后随着苞片基部的居间生长，有4～8束维管束在苞片内形成，但是新发育的维管束木质部和韧皮部相对位置与正常叶性器官一致，这与在以往报道的柏科植物成熟雌球果的苞片中均有反向维管束的发育不同。北美香柏雌球果早期发育和维管束分析结果支持傅德志和杨亲二提出的解释裸子植物生殖器官形态演化的“苞鳞-种鳞复合体”理论。%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 o vule primordium differentiated a collar-like integument around the base of the n ucellus, and the integument enclosed nucellus progressively and overgrew the nuc ellus, forming an ovule with a micropyle. During the ovule initiation, it was fu rther 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 orie ntation, which was reported as a feature in the cone development of Cupressaceae.