When plants developed the secondary growth, the almost unlimited perspectives opened for enlarging their body. However, these giants faced a new problem.
Some seed plants will excrete the drop of liquid from the top of the ovule (integument(s) + megasporangium), whereas the other, more advanced way is to grow a sperm delivery tool, the pollen tube (Fig...Some seed plants will excrete the drop of liquid from the top of the ovule (integument(s) + megasporangium), whereas the other, more advanced way is to grow a sperm delivery tool, the pollen tube (Figure \PageIndex4) made from one of the pollen grain cells. Figure \PageIndex4 Open fertilization in seed plants (left) with the help of liquid extracted by mother plant; and fertilization with pollen tube (right), or siphonogamy, with the help of pollen tube growing from male gametophyte.
