avena

If you cut the tip of a coleoptile, this will stop its growth which suggests that there is some chemical (auxin) present at the tip of coleoptile which controls growth. In order to know that whether the auxin is a water-soluble chemical, cut the tip of coleoptile and separate it with the rest of the coleoptile with a piece of agar. But coleoptile will grow and curve towards light (phototropism). This shows that auxin is water soluble, that’s why it passes through agar and reaches the tip.