双子叶植物茎进行次生生长时，维管形成层的射线状细胞向内进行分裂形成( )，向外细胞分裂形成( )。
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洋葱的食用部分是鳞茎，而构成鳞茎的肉质肥厚的叶片称为鳞叶。
“This element of the unknown”in the third paragraph refers to the fact that__________ ．
A. earthquake—related destruction has been beyond calculation
B. millions of people have died during earthquakes
C. earthquakes may occur anywhere at any time
D. there are other phenomena resulting from earthquakes
当花药发育成熟时，花药壁由表皮层和药室内壁构成，中层和绒毡层都消失了。( )
豆科植物分为三个亚科，但它们具有一些共同特征，即都具有蝶形花冠、荚果。( )
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