This general biological conception of hunger is probably as applicable to the lowest, or unicellular plants, as to the unicellular animals. But we have been unable to find any data bearing directly on this question. We have extensive studies on the chemotactic and general tropic behavior of the lower plants (Pfeffer, Kniep, Kusano, Shibata, etc.). Kniep ascribes senses of taste and smell to bacteria, and shows that the response of bacteria to certain chemical stimuli depends on the reaction of the culture medium, but he does not seem to have raised the question whether the quantity and quality of the food in the culture medium is also a factor in this response. Nor is the question of the effect of starvation on the behavior of bacteria raised in the recent extensive studies on the metabolism of bacteria by Kendall and his pupils.
The swarm cells of the spore-producing plants appear to feed by phagocytosis, just like the unicellular animals, and it is likely that this feeding phagocytosis in plants is influenced by hunger in the same way as in the lower animals. Lister describes the ingestion of microspores and bacteria by the swarm cells of mycitozoa, and records the following feeding behavior:
In one instance, after taking in two stout bacilli, and inclosing them in separate vacuoles, the swarm cell remained quiescent for a length of time. I watched the gradual process of digestion of the bacilli. After remaining (quiescent) under observation for nearly an hour and a half the swarm cell swam off with vigorous lashing movement of the cilium.
With the exception of some of the bacteria, the swarm cells of the sporophytes, the group of "parasitic" plants and, to a certain extent, the so-called "carnivorous" plants, the vegetable organisms feed primarily on the inorganic material in the soil and on the carbon dioxide of the air, and in these feeding processes motility of the plant, apart from growth, plays a minor part. The plant cannot move itself, nor can it move its food, even to the extent that both are possible in sessile animals. We may speak with perfect justification of starvation in plants, but in the case of the higher plants there is no evidence that starvation increases excitability and motility, that is, induces a biological state of hunger, nor would such changes aid the higher plants in securing food.