指种子植物个体发育中继子叶而生出的叶。在单子叶植物是夹着子叶和茎而位于相对一侧，在双子叶植物中则与两个子叶呈直角着生，也就是一个或两个对生。常常呈现与上部普通叶不同的叶形和叶序，多可理解为个体发育中表现出的系统发育的原始性状。
越冬卵孵化盛期为落花后至第一片叶展开之际，这是防治的有利时机。
The main period of egg hatching, which is the favourable opportunity for its control, lasts ho the flower fall- en to the first leaf Spreda out.
我们三个月前进军这里时，还是茫茫白雪，而现在，第一片春叶已经舒展开了。
When we marched in, three months before, the pjace was under snow; now the first leaves of spring were unfolding.
在播种后(萌前处理)或第一片真叶展开后(萌后处理)立即喷洒化合物。
Compounds were sprayed immediately after seed planting (preemergence treatment) or after the expansion of the first true leaf (postemergence treatment).
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