楼梯草属苞片形态和演化趋势

Morphology of the bracts of Elatostema(Urticaceae) and the evolutionary trends in them

(Ⅰ)对荨麻科楼梯草属的苞片和小苞片的形态进行了全面研究。(Ⅱ)该属原始群疏伞楼梯草组的雄聚伞花序苞片在每花序为15-90枚,纸质,绿色,狭卵形、狭三角形或条形,长0.5-4mm,扁平,无任何突起,而与楼梯草族的冷水花属和赤车属的聚伞花序苞片极为相似,因此,上述各种形态可以视为楼梯草属苞片的原始特征,并据此观察到总苞苞片的以下演化趋势:(1)近等大,形状相似,在花序托边缘轮生形成一层→排列为二层,外层2苞片对生,较大,内层苞片较小,形状稍不同;(2)狭卵形,狭三角形或条形→宽卵形或宽三角形,或扁半圆形→由于长度强烈缩小,宽度增大而最终消失;(3)扁平→顶端兜形→船形→船形,顶端突起成细筒;(4)无任何突起0背面有1龙骨状突起,或有1-6条纵肋或狭翅1顶端具短到长的角状突起→背面顶端之下具角状突起;(5)分生→基部合生→由于长度强烈缩小,宽度强烈增大,总苞苞片合生成一横条形狭片;(6)在数目上,由每花序的7-45枚,一方面增加到50-180枚,另一方面则减少到5枚以下。同时,观察到小苞片形态以下演化趋势:(1)膜质,半透明,白色0具褐色线纹或呈褐色→呈黑色1薄膜质,透明,无色;(2)扁平→顶端兜形→船形;(3)无任何突起→顶端或在背面顶端之下具角状突起;(4)在数目上,由每花序的7-45枚一方面增加到100或数百枚,甚至达1千到数千枚,另一方面则减少到5枚以下,甚至到0枚。上述演化趋势有助于了解属下各级分类群的演化水平。

（Ⅰ）The morphology of the bracts and bracteoles in the genus Elatostema is studied. （Ⅱ）The bracts of the staminate cyme in the primitive group of that genus,sect.Pellionioides,being 15-90 in number per cyme,papery in texture ,green in colour,narrowly ovate or triangular,or linear in outline,0.5-4 mm in length,flat,without any projection,are very similar to those of staminate cymes in the two genera of trib.Elatostemeae,Pilea and Pellionia,and their morphological features just mentioned above may be considered the primitive characters of the bracts in the genus Elatostema. On the basis of them the following evolutionary trends in involucral bracts are observed：（1）more of less equal in size and similar in shape,uniseriately verticillate on receptacle margin→arranged in two series,with two outer bracts opposite and larger,and the inner bracts smaller and more or less different in shape from the outer two;（2）narrowly ovate or triangular,or linear→broadly ovate or triangular,or depressed-semiorbicular→owing to reduction in size resulted from further strong decrease of length and increase of width disappearring at last;（3）flat→cucullate at apex→navicular→navicular,with a terminal cylindrical projection;（4）lacking any projection↗abaxially carinate,or longitudinally 1-6-costate or winged↘shortly to longly cornicullate at apex→corniculate abaxially below apex;（5）free→connate at base→owing to strong decrease of length and strong increase of width,the involucral bracts are connate into a transversely linear lamella around the receptacle margin;（6）increasing to fifty to one hundred and eighty from seven to forty-five in number per inflorescence on the one hand,and decreasing to less than five on the other. And the evolutionary trends in bracteoles may be showed as follows：（1）membranous,semihyaline,whitish↗brown-lineolate or brown→black↘thinly membranous,hyaline,colourless;（2）flat→cucullate at apex→navicular;（3）lacking any projection→corniculate at apex or abaxially below apex;（4）increasing to one or several hundred or even to one or several thousand from seven to forty-five in number per inflorescence on the one hand,and decreasing to less than five even to zero on the other. The evolutionary trends mentioned above may be helpful for understanding the evolutionary levels of various infrageneric taxa of Elatostema.