these traps is by no means clear, but minute crustaceans, &c., are fond of prying into holes and corners in search of food, while some of them may make use of the two branching bristles as a place of refuge from their enemies. And as the two bristles act as guides to the mouth of the trap the result is often tragic. Slimy hairs grow in this dangerous neighbourhood, which possibly have attractions to offer, while the little transparent door

Fig. 1068.-Traps of Bladderwort (Utricularia), enlarged. 1, External view. 2, Longitudinal section.
3, Three absorptive branched hairs from interior.

1 and 2, X 4: 3, X 250.

must look like a spot of light, and perhaps acts as a lure. It is at any rate certain that victims are numerous, and their decomposed remains are absorbed by curious branched hairs which

line the trap. The prey is apparently not digested as in Sundews

and Butterworts.

The remaining topics dealt with in this chapter also more or less involve questions of nutrition, but are placed under separate headings for the sake of clearness.

ASSOCIATION OF PLANTS AND ANIMALS AS MESSMATES, MUTUALISTS, AND PARASITES

COMMENSALISM. Two associated organisms are known as Messmates or Commensals when they live together to the benefit of one or both; the union, however, not being of so intimate a nature as to be essential to the life of either. The term commensalism was coined to express such relations as existing between different animals, but there appears to be no reason why its meaning should not be extended to cover cases where two plants, or a plant and an animal, are similarly related. As an instance of the former we may take those tropical Orchids which regularly live upon trees, and are on that account said to be Epiphytes (Gk.

epi, upon; phyton, a plant). The advantage to the Orchids is obvious, though they do not absorb the sap of the plants upon which they live. These last, however, apparently derive no benefit from this one-sided arrangement.

There are numerous cases of commensalism between plants and animals in which the latter alone are benefited. In one of the Liverworts (Frullania dilatata, fig. 1069) which grow on tree-trunks there are little cup-like outgrowths on the under sides of the leaves, serving as the abodes of a species of Wheel-Animalcule (Callidina symbiotica). Marine plants often bear animals as messmates, which do them no harm. On a kind of brown sea-weed (Fucus serratus), for example, are frequently to be seen the little spiral tubes of a sort of Annelid (Spirorbis), which no doubt secures an increased supply of nutriment and dissolved oxygen by being moved about in the water when the tide is up. The Australian Sea-Horse (Phyllopteryx eques) also benefits by its association with the sea-weeds to which it bears a resemblance (see vol. ii, p. 296).

Fig. 1069.- Piece of a Liverwort (Frullania dilatata), showing cups in which a Rotifer (Callidina symbiotica) lives: enlarged. The small drawing is of one cup with its Rotifer (further enlarged).

Some instances are known of commensalism between a plant and an animal, in which both derive advantage from the association. Ant-plants illustrate such an arrangement (see p. 81), and so do the Sloths of South America, in which minute algæ live in the grooves of the fluted. hairs. For these algæ are provided with a sheltered home, and at the same time give a greenish tint to the hairs, the Sloths being thereby rendered less conspicuous to their enemies. MUTUALISM (Symbiosis).-Organisms living together as Mutualists are very intimately associated for mutual benefit. Mutualism between two plants is well illustrated by leguminous forms and the minute fungi which live in the tubercles on their roots. And every Lichen may be regarded as a joint-stock community, con

sisting as it does of an alga and a fungus closely interwoven (fig. 1070).

Animals and plants may also be associated in an intimate way. It appears, for example, that the process of digestion in many animals is aided by certain bacteria which always live in their internal organs, as, e.g., Sarcina ventriculi in the human stomach (fig. 1073). Bacteria of the sort are provided with a sheltered home and abundant food. Among the Animalcules (Protozoa) a well-known example is afforded by some of the Radiolaria, which always contain so-called "yellow cells", that

3

Fig. 1070.-Cross-section through a Lichen (Collema), showing the colourless threads of the Fungus, and the dark necklace-like filaments of the Alga. X450.

are regarded as a kind of alga (fig.
1071). These cells are not only shel-
tered, but also absorb carbon dioxide,
water, and salts from the fluids of the
Radiolarian, which in its turn is pro-
vided with abundant free oxygen for
breathing purposes, and possibly bene-
fits in other ways.
A somewhat similar
association between some Sea - Ane-
mones and minute algæ has been de-
scribed. It is, however, possible that
"yellow cells" and "algæ" are not
plants at all, but specialized parts of the
Ray- Animalcules and Sea-Anemones
themselves.

PARASITISM.-An organism is known

as a parasite when it feeds upon the substance of another organism, to the serious or fatal detriment of this unwilling "host". An ectoparasite lives on the outside of its host; an endoparasite

within it.

Many plants prey upon other plants in one way or the other. Clover- Dodder (Cuscuta), for example, is ectoparasitic upon Clover, while various fungi live as endoparasites within higher plants, e.g. Potato-Fungus (Phytophthora infestans) within the tissues of the Potato plant.

A large number of plants are known which are endoparasitic with regard to animals. In autumn many dead flies will be seen adhering to various objects by a sort of fluffy halo which surrounds them. These have been killed by the Fly-Mould (Empusa musca, fig. 1072), the delicate threads of which branch in

all directions through their tissues. Other fungi attack various caterpillars, e.g. the silk-worm disease known as "muscardine" is due to the Silk-worm Mould (a species of Cordiceps). A number

of skin-diseases, such as ringworm and "barbers' rash" are caused by parasitic plants of somewhat similar nature.

But the most notable, and at the same time the smallest, of the endoparasitic plants which attack animals are certain kinds of bacteria, which may literally swarm within the body, and give rise to a host of diseases, such as relapsing fever, typhoid, leprosy, Asiatic cholera, tuberculosis, diphtheria, anthrax, lock-jaw, and bubonic plague. Some idea of the small size of bacteria will be gathered from fig. 1073, or from statements that make some appeal to the imagination. It is said, for example, that 250,000,000 individuals of the species associated with bubonic be crowded into the small space of a square inch. A number more than six times as great as the population of the United Kingdom at the last census.

(Musca domestica) killed by Fly-Mould (Empusa

Fig. 1072.-A House-Fly

musca), enlarged

plague could

In dealing with those animals that feed upon plants it is impossible to draw any clear line between vegetarians and parasites. We shall, however, be justified in applying the latter name to a number of small forms which live, generally for part of their lives only, within the tissues of plants, one consequence being the formation of certain abnormal growths. Some of these will be dealt with later on in connection with the subject of agricultural pests. The clubbing of turnip-roots ("finger-and-toe" or

Fig. 1073.-Bacteria. 1, The "blood portent ' (Micrococcus prodigiosus); 2, gelatinous stage of the same. 3, Bacteria which produce acetic acid (Bacterium aceti); 4, the same on larger scale. 5, Bacteria of Asiatic cholera (Spirochate cholera asiatica); 6, the same on larger scale. 7, Anthrax bacilli (Bacillus anthracis) with red blood-corpuscles; 8, the same on larger scale. 9, Bacteria of relapsing fever (Spirochete Obermeieri) and red blood-corpuscles. 10, Symbiotic bacteria (Sarcina ventriculi) from human stomach. 1, 2, 3, 5, 7, and 9, X 300. 10, x 800. 4, 6, and 8, x 2000.

"anbury "), for example, is caused by one of the Fungus-Animals (Plasmodiophora brassica), which interferes with the nutrition of the plant, causing it to grow in an unusual way. And it not infrequently happens that cereals and some other cultivated forms are attacked by small Eel-Worms, the presence of which has a stunting or distorting effect.

Most persons have noticed the curious local outgrowths known as "galls" that are common upon some plants, and tempt comparison with the tumours and cancers of animals. They are due to the attacks of Gall-Flies, small forms belonging to the order of Membrane-winged Insects (Hymenoptera). The female gall