88 THE VEGETATIVE FUNCTIONS OF PLANTS 
Hum of the embryo. Often this glandular layer is in- 
vaginated, so as to form a true gland (Fig. 62). 
A special case of plant nutrition is presented by the 
insectivorous plants, of which there are many kinds. 
They are all characterized by the possession of some de- 
vice fbr capturing insects that visit them, and by the 
ability to 'secrete a proteolytic enzyme capable of digest- 
ing the protein tissues of their prey. These plants can 
also digest any meat fed to them artificially. After 
being digested the protein food is absorbed by osmosis. 
The Venus's flytrap (Dioncea muscipula), numerous 
'species of bladderwort (Utricularia), and several species 
of sundew (Drosera) serve to illustrate the insectivorous 
plants (Figs. 63-65). Although these plants are able 
to digest protein, experiments have shown that a pro- 
tein diet is not necessary either for their life or healthy 
growth. 
86. Assimilation. After food has been elaborated and 
digested it still is not a part of the living protoplasm, but 
only lies within the vacuoles of the protoplast. It is no 
more a part of the plant than is food in our hands or 
stomachs a part of our bodies. One step more is nec- 
essary; the digested food must be incorporated into, and 
made part of, the living protoplasm itself. It must be 
transformed from lifeless matter into living matter. As 
in the case of enzyme action, the process by which this 
remarkable change is brought about is not understood. 
By multitudes of accurate, painstaking experiments, 
however, one great fundamental truth has been established, 
namely, that non-living matter can be converted into 
living matter only by the action of other living matter 
already existing. Proteins, sugars, and fats have all 
