not readily absorb colouring matters, consequently in this state is not easily stained. Bismarck brown, a colouring matter soluble in water, if added in very small quantities,

Fig. 1.-A, part of the naked protoplasm of Didymium leucopus, a Myxogaster in the motile condition during the vegetative state; the strands of protoplasm are continually changing their form, and the whole mass also moves (× 300). B, a closed sporangium of another species of Myxogaster, Arcyria incarnata; C, the same after rupture of wall of sporangium p; cp, the expanded capillitium (x 20). (After Cienkowski, from Prantl.)

will however stain the protoplasm of living organisms without in any obvious manner interfering with the usual routine of life, at least for some time. Dead protoplasm,

on the other hand, greedily absorbs colouring matters, and usually becomes much more intensely coloured than surrounding structures. Owing to the varying composition of actively living protoplasm, no reliable chemical test for it is known, in fact, the tests usually given depend on the presence of various substances which are assumed to be constantly produced by the activity of living protoplasm. It becomes blackened after having been in contact for some time with a very dilute solution of nitrate of silver (even one part in 1,000,000 parts of water); this test proves the presence of living protoplasm, the reduction of the silver-solution being due to the reaction of the albumen of living cells, the failure of this reduction in dead cells is due to a chemical change having taken place in the albumen. Molybdic acid gives to dead protoplasm a deep blue colour. Very little is known respecting the structure of living protoplasm. It has recently been described as consisting of very delicate, spirally twisted, hollow tubes, composed of a colourless, unstainable, somewhat gelatinous substance, that easily swells up in water. These threads are in turn spirally coiled and form the walls of hollow cylinders; the hollow tubes and the cylinders formed by them are filled with the so-called granular protoplasm, in which the streaming or movements take place. The spiral threads and the cavities they form are the portions of physiological value, and these are not isolated in each cell of the organism, but continuous from one cell to another throughout the entire structure.

The relation of life to surroundings has to some extent been already demonstrated, nevertheless other far-reaching or universal dependencies on the nature of the environment require attention.

(a) Food is the only source of material by which a living

body is enabled to add to its bulk, or grow, and at the same time to replace those portions of its own living substance that disappear in proportion to the amount of work done, chemical and physical, collectively known as vital energy. Hence it follows that one important factor in determining the presence or absence of life at any given spot, would consist in the presence or absence of food material under a form suited to the requirements of life. Keeping in view the well-known fact that different forms of life require different kinds of food, chemically considered; and further, that the physical condition of the food is also of importance, we gain a further insight as to the conditions that determine the presence of a given group of organisms at any particular spot. As an illustration of the above statement, assuming for the moment food to be the only factor in determining the distribution of life, we should find that a moss could live under conditions where it would be absolutely impossible for a fungus to do so for the following reasons. The food of a moss plant consists of inorganic matter, that is, matter whose existence in its present condition is not directly due to life; carbonic dioxide, derived from the atmosphere, and water along with small portions of various substances derived from the soil dissolved in it, and absorbed by the root of the moss, furnish it with all the chemical elements which its own vitality enables it to rearrange and convert into its own substance. A fungus, on the other hand, does not possess the power of decomposing and utilizing inorganic matter as food, but can only feed on organic matter, that is, matter that owes its present chemical composition to the direct action of life; consequently, other things being equal, the moss has the choice of a much wider range in which to

establish itself, on account of the almost universal presence of its food, than is the case with the fungus. In the nature of its food the moss may be regarded as typical of the great majority of plants, in fact of all those that are green; this colour, called chlorophyll, being inseparably connected with the assimilation of inorganic food; but it is important to bear in mind that although all green plants agree with the moss in requiring carbonic dioxide as one food constituent, yet all are not equally indifferent as to the nature of the food substances absorbed in solution in water, an unusual and sparsely-distributed substance being in some instances an indispensable food constituent, consequently we find that the nature of the soil, or the geological formation of a given district, determines to a great extent the nature of its own flora. The fungus, in the nature of its food, is characteristic of all those plants not furnished with chlorophyll, agreeing with the members of the animal kingdom in requiring organic matter as food. But in the fungi we find that food determines to some considerable extent the distribution of the species. Many kinds of fungi, so far as can be ascertained, have no marked partiality for any particular kind of food; other conditions being favourable, so long as organic matter is present, such fungi generally appear. On the other hand, some kinds of fungi are so restricted in the matter of food that they are confined to being parasitic on the leaves of one particular species of plant; consequently, in such instances the limits. of distribution of the fungus is necessarily influenced by the range in space of the plant upon which it is dependent for food.

(b) Moisture. The manifestation of life in the active state is absolutely dependent on the presence of water in

the liquid condition. The following, amongst other reasons, explains this statement: the various substances constituting plant food are brought into contact with the protoplasm through the agency of water in which they are held in solution; this substance also conveys assimilated food from its point of origin to those portions of the plant that require food, but cannot prepare it directly for themselves. In the passive condition of plant life, the amount of water present is reduced to the minimum, but is probably never entirely absent.

(c) Temperature.-The vital functions of plants appear as a rule to be confined between the temperatures o° C. (=32° F.) and 50° C. (= 122° F.); but the same functions have very different limits between these two extremes in different plants, and even in the same individual different functions have different limits. Gourd seeds will not germinate at a temperature below 13° C. (=55'4° F.), nor at one above 42° C. (= 107·6° F.), whereas in barley the limits to germination are 5°C. ( = 41° F.) and 37°C. ( = 98·6° F.). Between the two extremes of temperature at which any function can be exercised there is one point, the optimum temperature, at which that function is performed with the greatest activity, and any departure from this point in the direction of the maximum or minimum becomes less and less favourable for the performance of that function.

(d) Light is an indispensable factor in connection with green plants, inasmuch as the development of chlorophyll is dependent on light; and further, the work done by chlorophyll can only be exercised under the influence of light. Various other forces less general in their influence on plant life do not call for special notice at present.

It must be clearly recognized that favourable conditions in