The bulb on Ivysaur&#39;s back grows and strengthens itself by absorbing solar energy. The exposure to this sunlight also strengthens the Ivysaur. As the bulb grows, Ivysaur&#39;s legs grow sturdier and as such the Ivysaur loses the ability to stand on its hind legs.
<br /><br />
Ivysaur that are close to evolution can frequently be seen sunbathing. The bulb of an Ivysaur close to evolution also gives off a pleasantly sweet aroma.