大关杨树皮的形成与季节变化的关系

大关杨的输导韧皮部限于当年新生的次生韧皮部。４月中旬至５月中旬大量出现，７月底停止发育。其成熟筛分子中具一细胞核，以后才消失。前一个生长季产生的筛管变形，故二个生长季的韧皮部蛤限明显。韧皮纤维出现迟，呈切向束状分布。含晶细胞分布于纤维束两侧。木栓形成层５月中旬开始活动，７月进入发育高峰。

THE RELATIONSHIP OF THE BARK FORMATION WITH SEASONAL CHANGES IN POPULUS DAKANENSIS

It is discovered that the conducting phloem is the secondary phloem produced during new growth period in P. dakanensis. It develops quickly from mid-Api to mid-May during new leaf-growing and shoot-elongting period and ceases formation by the end of July. There is a nucleus near the sieve plate in the mature sieve element. The nucleus disappears later. The sieve elements of preceding growth period are damaged, collapsed and losetheir conducting function during new growth period so that distinction is obvious between the phloem of preceding and that of new growth period. The phloem fibers are produced after the full development of the sieve elements, companion cells and perenchyma ceils. They distribute in tangential bands during new growth period and encircle when the parenchyma cells between the bands in the non-conducting phloem differentiate into sclerieds and fibers next year and there is a sclerenchyma ring on the transection. Crystal-containing cells distribute in both sides of the fiber band. Square crystals deposit from small to big. The activity of phellogen is from mid-May and the full development is in July. Annual ring of bark is suitable for the bark of the shoots in P. dakanensis.