Chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells.
Fungal and phanerogamic parasites can make no use of stich substances as carbon dioxide, but draw elaborated products from the bodies of their hosts.
If we examine the seat of active growth in a young root or twig, we find that the cells in which the organic substance, the protoplasm, of the plant is being formed and increased, are not supplied with carbon dioxide and mineral matter, but with such elaborated material as sugar and proteid substances, or others closely allied to them.
This is the absorption of elaborated compounds from their environment, by whose decomposition the potential energy expended in their construction can be liberated.
This source is not, however, anything new, for the elaborated compounds so absorbed have been primarily constructed by other plants through the mechanism which has just been described.