400 



THJ^; GARDEN AND FIELD. 



February, 1914 



Plant Growth. 



When a seed is placed in a suitable 

 environment as regards warmth and 

 moisture it absorbs the latter, swells, 

 and softens; the starchy matter stored 

 up for the nutrition of the young plant 

 becomes soluble, and the radicle or 

 future root descends into the soil, 

 whilst the plumule ascends to form the 

 stem of the plant. It is at once clear 

 that the formation of a good seed-bed 

 allows the penetration of the young 

 roots into the soil, and the depth of 

 planting the seed should be such that 

 the first young leaf-shoot will be above 

 ground before the store of food pro- 

 vided in the first place in the seed is 

 exhausted. In the case of plants which 

 only live one year there is a marked 

 activitj' of growth of the root and 

 leaves, which collect, and with the aid 

 of sunlight prepare material for the 

 growth of the plant. The leaves are the 

 centres of activity where the materials 

 from the roots meet the carbon from 

 the atmosphere, and the various ma- 

 terials required for the nourishment of 

 the plant are manufactured. The next 

 step in the case of annuals is the forma- 

 tion of the flower, which is followed 

 by seed, after which the plant dies. 



The composition of a plant, as 

 above-mentioned, varies at different 

 periods of its existence. In the early 

 stages the nitrates and mineral matters 

 taken in by the water absorbed through 

 the root hairs are in excess, but as 

 growth proceeds the percentage of car- 

 bonacoeus materials derived through 

 the leaves steadily increases. When 

 the crop is in full bloom it ceases to 

 take in any more nitrogen and potash, 

 although the absorption of phosphoric 

 acid continues a little longer, but the 

 assimilation of carbon through the 

 medium of the small openings or 

 stomata of the leaves will continue as 

 long as the plant remains in a green 

 condition. When the process of form- 

 ing seed commences the whole of the 

 energies of the plant appear to be con- 

 centrated in the direction of transport- 

 ing the materials (starch, albuminoids, 

 &c.) from the root, stem, and leaf, and 

 depositing same as seed. When the life 

 history of the plant has been uninter- 

 rupted, and what is known as a good 

 season prevails, this migration of ma- 

 terial from the vegetative portions of 

 the plant will be very complete, and the 

 straw of a cereal crop at harvesting 

 will be found to be very completely 

 depleted of foodstuff. Should, how- 

 ever, the crop fail to reach maturity. 



or in seasons of deficient sunshine and 

 poor seed-formation, the material as- 

 similated and manufactured during the 

 growth of the crop remains in the 

 straw. It is thus quite obvious that 

 cutting the crop before the seeds have 

 been completely formed is the most 

 certain means of getting a straw of 

 high-feeding quality . 



In dealing with plants which live 

 longer than one year a diflferent order 

 of things prevails. The same marked 

 activity of growth of the vegetative 

 organs (roots and leaves) is observed, 

 but at the end of summer, instead of 

 the formation of seed, the matter col- 

 lected during growth is stored up in the 

 roots, tubers, or stem. The next season 

 the plant throws up a flowering stem, 

 and the foodstuff thus accumulated is 

 then used for the production of seed. 

 This is the cycle as regards biennial 

 plants, such as mangels, &c. 



Again a difference is met with in the 

 case of trees and shrubs which last for 

 years, and which do not form fleshy 

 receptacles. Here we have a trunk of 

 more or less thickness and stout roots 

 in active growth for a greater part of 

 the year. Between the bark and the 

 wood is a layer of cells called the cam- 

 bium layer, which is constantly divid- 

 ing to form fresh wood and bark, and 

 running at right angles to the pith of 

 the tree are also channels of cells 

 called the medulary rays. It is in these 

 regions that material is placed to supply 

 the necessary activity to perennial 

 plants when active growth is taking 

 place. 



Tt will now be of interest to show 

 briefly what materials are first employed 

 to serve as nutritive matter for plant 

 growth, and by what agency such com- 

 l)inations are brought about. By grow- 

 ing plants in sand free from any or- 

 ganic matter it has been demonstrated 

 that large quantities of carbon and 

 nitrogen are obtained, which, together 

 with the elements of water and the 

 mineral salts imbibed with the water, 

 constitute the organic material of 

 plants. Now. where did the carbon 

 come from? As the sand did not con- 

 tain any humus it is quite clear tliat 

 the source of supply must have been 

 the atmosphere. In the air there are 

 a1)out three parts of carbon dioxide in 

 10,000. This, though only a very 

 minute quantity, suffices, as the leaves 

 as compared with their weight offer 

 an enormous absorbing surface, and are 

 being constantly agitated by the wind, 

 which brings a large quantity of carbon 

 dioxide into contact with the plant. 



Now, providing the plant is in a normal 

 condition, that is, the leaf cells are filled 

 with water, and are constantly being 

 supplied with moisture ijom the rootsi 

 to replace that lo.st by transpiration, the 

 orange and yellow rays from the sun- 

 light acting on the green colouring 

 matter in the cells will cause the carbon 

 dioxide absorbed in the water of the; 

 plant cell to decompose, the oxygeni 

 being liberated whilst the carbon is re- 

 tained. The carbon unites with the ele- 

 ments of water in the cell, and we have 

 the simplest primary compound known 

 as formic aldehyde. By a simple pro- 

 cess of condensation this material would 

 be clianged into one of the sugars. As 

 will he seen from the above, light .is 

 absolutely* essential to the assimilation] 

 of carbon by the leaf cells, and this fact 

 is clearly borne out by the rapid up- 

 rush of cereals in northern countries, 

 the long period of light allowing un- 

 interrupted growth to take place. 



Starch is one of the earliest products 

 of assimilation, and is very abundant in 

 leaves exposed to sunlight, but at the 

 end of the day it disappears, being 

 changed into some form of sugar by 

 the action of an unorganised ferment, 

 or erzyme. By this means the starch 

 is conveyed by the sap to distant parts 

 of the plant where growth is actively 

 going on, or else it is reconverted into 

 starch and deposited, as in the case of 

 the potato, in the tuber. 



Whilst it is comparatively easy to 

 explain the formation of carbohydrates 

 in plants, the manner in which the 

 alliuminoids, vegetable acide, and fatty 

 matter are produced is not so clear. 



According to Warrington, the mode 

 in whicli alliuminoids are formed is pos- 

 sibly that tlie nitrates taken up by the 

 roots are converted into ammonia, the 

 ammonia into amides, and the amides 

 finally into albuminoids. These changes 

 were clearly shown by the investigations 

 of Wood at Cambridge, when searching 

 for an explanation of the changes in the 

 comjiosition of mangolds during stor- 

 age. When pulled up in the early 

 autumn the roots were found to be full 

 of nitrates — the form in which nitro- 

 gen is first taken up by the roots of 

 plants. A few months later it was 

 found on analysis that these nitrates 

 had been largely changed into amides, 

 and there was also an increase in the 

 albuminoids and peptones. 



♦ 



A house without a g'arden is 

 like a pboto frame without a pic- 

 ture to set it off. 



