i68 



I'HE TROPICAL AGRICULTURIST. [September i, 1885, 



of the functions of the chlorophyll, but like many other 

 known facts in science, the reason of this has not yet 

 been satisfactorily explained. 



Irmi has been found to be necessary for the proper 

 formation of chlorophyll, it having been observed that 

 plants which have become pale, may, Uke human beings 

 under similar circumstances, have their colour restored 

 by the administration of some preparation of iron, and 

 that no green colour can be produced without the 

 presence of iron in the soil. The cause of this is, however, 

 unknown. 



Macjnesium and Sodium also appear to be essential elements, 

 although no special function has as yet been assigned to 

 them. 



Silicon is to be found in the stems of such plants as 

 grasses, and in the cell wall of diatoms, in which the 

 function appears to be the stiffening and strengthening 

 of the parts. It is doubtful, however, whether or not 

 silicon is really essential to the plant, as plants are known 

 to be perfect without the presence of silicon, and the 

 prevading opinion appears to be that its presence in 

 plants is, to a great extent, at least, accidental. 



Chlvfine has been found to be present, but as plants 

 can grow equally well without it, it is probably accid- 

 ental also. 



Iodine is also found in sea plants, which form one of 

 the chief sources of that substance. 



Fluoriae is also supposed to be present, as it is found 

 iu herbivorous animals. 



Phosphoric Acid appears to stand in a certain relation 

 to the production of albuminoids. 



Calcium seems to act as a vehicle for sulphuric acid, 

 and also for precipitating the oxalic acid continuously 

 being formed, and which is injurious to the plant. It 

 is thus that crystals of oxalate of lime are so abundant 

 in certain parts of the plant. When plants are grown in 

 fluids which contain all the substances essential for their 

 life, it is found that the substances in solution are not j 

 taken up indiscriminately, but that different species of 

 plants supply their wants in different ways. Plants, there- 

 fore, appear to have the power of selecting for themselves 

 the substances necessary for their growth. LeguminosK 1 

 are specially dependent on lime, potatoes and turnips on i 

 potash, and cereals and grasses on sihca, j 



It is on this fact that the principal of rotation of crops 

 is founded. During one year, a certain kind of grain 

 exhausts the soil of its own inorganic materials, requiring 

 next year a crop which feeds on different salts, and so | 

 on until the soil has again been replenished from other 



The system of manuring also depends on the principle 1 

 that plants require different inorganic salts to feed upon. 



2. The Marnier in which this Food is ohtained. — The organ 

 which is chiefly concerned in the nutrition of the plant 

 is the root. This organ may be defined as that part 

 of the plant which fixes it in the substratum from which 

 it derives nourishment. This definition does not apply 

 in both particulars to all plants, some roots, as those 

 of seaweeds, being solely for fixation, and some, as in 

 the duckweed, simply for nutrition. AVe have, however, 

 at present to deal with only the nutritive functions of 



In"^ most text-books it is stated that absorbtion takes 

 place only at the tip of the root, which has, on that 

 account received the name of " spongiole." It has since 

 been observed that this spongiole now called the " rootcap, 

 acts simply as a sheath to protect the growmg pomt 

 which lies immediately behind it. It is the hairs on the 

 eiiidermis of the root, and these only, which take in 

 nourishment. These hairs are outgrowths of the epidermal 

 Cells and, like all other cells, are filled with protoplasm 

 ajul 'cell-sap. Being very delicate, these hairs are easily 

 affected by gravitation, so as to lie alongside all the 

 panicles of soil in their vicinity. The moi.-,ture adhering 

 to these particles of soil is absorbed into the hair through 

 the thin cellulose wall, by a process known as endosmose. 

 This process is based on the fact that when two Uquids 

 of different densities are separated by any thin organic 

 membrane they tend to change places, the less dense 

 liquid passing through the membrane proportionately faster 

 than the denser one. In the case before us we have the 

 water in the soil separated from the more dense celli 



sap by the thin membrane of the root-hair, and thus by the 

 law of endosmose the warter enters the root-hair more 

 quickly than the cell-sap comes out. The water thus 

 absorbed contains, in solution, the various inorganic salts 

 necessary for the life of the plant. Carbonate and sulph- 

 ate of lime and other salts, insoluble in water, are 

 dissolved by the oxalic acid which is always present in 

 the cell sap, from causes already mentioned, and are thus 

 absorbed. In aquatic plants, as the greater part of the 

 plant is entirely surrounded by water, it requires 

 few, if any, special orgaus for the absorption of fluid. 

 The roots are therefore either rudimentary or entirely 

 absent. In the duck-weed, for example, the root consists 

 of single long thread, furni.shed with a root-cap at 

 the end. Absorption takes place from the whole surface 

 of this root, there being no root-hairs. The roots of some 

 orchids which grow on the branches of trees never touch 

 the ground, but derive all their nourishment from the 

 air. Such plants are called " epiphyte.s." The outer cells 

 of the root are called " velameu," and have the property 

 of absorbing watery vapour from the air. That class of 

 plants called •' parasites " also deserves passing notice. 

 These do not derive nourishment from the ground, but 

 get it from a *'host." Take, for example, a plant which 

 will acquire great importance in society dm'ing the next 

 few weeks — the mistletoe. This, however, is only a partial 

 parasite, because, having green leaves of its own, it requires 

 to take in from its " host " only water containing mineral 

 matter in solution, this sap being elaborated iu the leaves. 

 The root is of peculiar construction, one tap-root decending 

 directly into the wood, while two lateral adventitious 

 roots lie along between the wood and the bark. 



The dodder is, perhaps, the best example of a parasitic 

 plant. Unlike the mistletoe, it spends part of its life 

 growing in the ground in a normal fashion. In this state 

 it consists of a root and a lonjj slender stem, the end 

 of which gropes about till it finds a plant (preferably 

 clover) to which it fastens itself, and at the expense of 

 which il lives all the rest of its life, the other part of 

 the stem, together with the root, dying away. 'When the 

 stem finds a host, it coils round it and sends out brush-like 

 roots to the bast of its host, whence it derives nourish- 

 ment ; in this case, however, fully elaborated and ready 

 for use. 'When its host dies, the dodder finds its way 

 to another, and often spreads so rapidly as to ruin a 

 whole clover field in a short time. 



Having considered the manner in which the fluid is 

 absorbed by the root, we must now consider how it is to 

 be conveyed to the various parts of the plant. AVe have 

 already seen, that about 75, per cent by weight of the 

 plant consists of water, and that, without that element, 

 life is impossible. In many plants there is also a con- 

 tinual loss of water from the leaves by evaporation. To 

 meet these requirements it is therefore necessary that 

 currents of water should continually pass through the 

 plant in all directions. The old theory of -'ascending and 

 descending sap" is therefore untenable, as the sap has 

 to go wherever it is required, which is sometimes up, 

 sometimes down, and sometimes in a lateral direction, 

 finally passing out of the plant in the form of vapour by 

 " transpiration." The causes of these movements are cheifly 

 capillarity and endosmose, but there is also a process of 

 suction caused by the evaporation of water from the leaves, 

 and also another force called "root-pressure." When the 

 roots absorb more water than plant requires, this exercises 

 a pressure which drives the water further up the stem. 

 This water exudes on the margins of the leaves, and as 

 it of ten gathers on the leaves during night, it is fro()uently 

 mistaken for dew in the morning. The phenomenon of 

 bleeding, which is seen in trees which have had their branches 

 cut off in spring, is owing cither to root-pressure, or to 

 the expansion of air in the phmt, caused by variations 

 in temperature, thus driving out the water wherever there 

 is an outlet. 



We have now arrived at the most iraport.ant part of 

 our subject, viz., the processes of assimilation and meta- 

 stasis. ^Assimilation is the conversion of the crude materials 

 into substances, such as starch, which may either be 

 used at once for the nutrition of the plant, or stored 

 away for future use. metastasis is the conversion of these 

 '•resetvo materials" into new cells, and also into what 



