258 



THE AGRICULTURAL NEWS, 



AirOxUST 20, 1910. 



one of them will prevent its proper development from 

 taking place. These conditions are called limiting 

 factors, because each of them alone is always, and 

 absolutely, necessary to the growth of the plant. Fur- 

 ther, as is stated in a recent article * that deals with the 

 subject, and which may be consulted with advantage, 

 the factors requisite for the life of a plant mostly act 

 togetlier throughout its whole existence: they do not 

 show their intluc-nce successively. The result is that the 

 partial absence of any one of these factors, at any time, 

 only allows the others to exert their influence to an 

 amount that is permitted by the degree to which it is 

 present; while its total absence completely prevents 

 them from being useful in any way. 



It will be well, now, to consider the nature of these 

 essentials that must be satisfied simultaneously, and 

 each in its proper degree, before the growth of green 

 plants can continue. They are, in order of immedi- 

 ate urgency: (1) the presence of water; (2) a certain 

 range of temperature: (3) a supply of mineral salts; (4) 

 the presence of certain kinds of light; (5) air containing 

 oxygen and carbon dioxide. These will be taken in order. 



The necessity of water to plants is that which is 

 most readily recognized, and the want of this essential 

 is most quickly shown by them. If it is considered alone 

 that man is dependent upon the rainfall for this requi- 

 site, it is easy to regard him as being helpless in its 

 absence or insufficiency, owing to the failure of the latter. 

 That this view of the matter is not justified is made 

 evident from a review of the progress that has taken 

 place in matters of irrigation, and what is more 

 important, in that of the treatment of the soil for 

 the purpose of conserving the amount of moisture that 

 it contains already. The knowledge that has been 

 gained concerning surface tillage has opened up, for 

 the agriculturist, parts of the world that were formerly 

 considered of too arid a nature ever to be of any use to 

 him; and it has given him, in places subject to inter- 

 mittent droughts, a means of saving the water in the 

 soil, for the uses of his crop, so that he is assured of 

 some return for his toil, even in circumstances under 

 which he would have previously despaired. The impor- 

 tance of the supply of water to plants will be realized 

 all the more clearly in the apprehension that its insuffi- 

 ciency or absence lessens or destroys the usefulness ol 

 all the other factors. However rich the soil may be, the 

 plants in it can only make use of as much of the food 

 that it contains as is permitted by the proportion to 

 which their needs for water are satisfied. Artificial 



* The Journal of the Department of Agriculture of Victoria, 

 Vol. VIII, p. lioS. 



manures, in the absence of sufficient water, are wasted 

 in a large degree, for the immediate crop. They are, 

 indeed, directly harmful, in any quantity, for they make 

 it less ea.sy for the plant to absorb what water is pres- 

 ent, by stunting the growth of the roots, and by increas- 

 ing the strength of the soil water solution, so that the 

 efficiency of the toot hairs in taking it up is seriously 

 impaired. 



Most green plants show distinct preferences in the 

 matter of temperature; this is the most powerful factor 

 in regulating the arrangement of the different kinds 

 over the surface of the earth. In temperate climates, 

 the distribution of heat or cold throughout the year is 

 of the greatest importance to the agriculturist. In the 

 tropics, it only requires consideration in relation to 

 the possibility of the introduction and acclimatization 

 of plants : it is always sufficiently high for the needs 

 of those which are indigenous. 



The quantity of mineral salts that is necessary to 

 plants is very small; where these are present abundant- 

 ly, however, the. plant will make use of much more than 

 the requisite minimum, with a probable increase in its 

 development. The concern of the practical agriculturist 

 is, naturally, most particularly connected with the 

 knowledge of means to supply the essential amounts of 

 this kind of plant food. It is a matter of interest 

 that some mineral constituents, if they are deficient, 

 can be replaced to some e.xtent by others: examples of 

 this substitution are magnesium for calcium, and silica 

 (in cereals) for phosphoric acid; it is assisted to be of 

 use to the plant, by the power of selection that the 

 latter possesses — a power which enables it to make the 

 best of what is at its disposal. Short mention, only, is 

 required of the fact, that the agriculturist can come to 

 the assistance of the plant with arrangements for 

 fallowing, rotation and the employment of artificial 

 manures. 



It is a matter of common knowledge that green 

 plants require light, though they do not succumb, for 

 some time, on being deprived of it. The necessity for 

 light is bound up with that for air, and the two can 

 be best considered together. A plant deprived of air 

 would most quickly suffer for want of oxygen, which is 

 necessary to it, and which it can use whether light is 

 present or not. The case is different in regard to 

 carbon dioxide, for this can only be used with the aid 

 of light. Under ordinary conditions, the former requi- 

 site is always present, but it is of no use to the plant 

 unless the right kinds of light are available to assist 

 in its assimilation. Practical considerations rarely 

 require to take account of these essentials, but they are 



