For example , the basic angiosperm stem is vertical and elongated , but some cacti have swollen water - storing stems , while other xerophytic plants that have lost their leaves during the course of evolution have evolved flattened leaflike stems for photosynthesis , e . g . butcher ' s broom ( ruscus aculeatus ) 例如被子植物的茎是垂直的，伸长的，但一些仙人掌具有膨大的能够储水的茎，另一些旱生植物在进化的过程中失去了叶子，但又发展为利用扁平叶状的茎来进行光合作用（假叶树） 。
