516 



GARDENERS' CHRONICLE 



mass comprises cells containing numerous minute green bodies 

 (known as chlorol'hsts) containing chloropliyll which give color 

 to the whole leaf, although under the microscope they are seen 

 to be quite distinct from the cell-protoplasm; but as this dis- 

 tinction is invisible to the unaided eye the green color appears to 

 pervade the entire leaf, more especially as the chloroplasts are 

 arranged along the sides of the cells nearest the outside so that 

 they can get more light. 



The cells, of which the leaf tissue is made up, are usually 

 arranged differently in the upper and the lower sides of the leaf. 

 In the upper part the cells just beneath the epidermis are elongated 

 at right angles to the surface of the leaf and stand in close 

 contact, forming what is known as the palisade tissue. In the 

 lower region of the leaf the cells are irregular in form and so 

 loosely arranged as to leave air spaces between the cells, the 

 whole' region forming a spongy tissue. The air spaces communi- 

 cate with one another, thus creating a labyrinthine system of air 

 chambers throughout the spongy tissue. It is into this system of 

 air chambers that the stomata open, and thus what may be called 

 an internal atmosphere is in contact with the working cells 

 containing chlorophyll, and this internal atmosphere is therefore 

 in full communication through the stomata with the outside air. 

 The white areas of variegated leaves usually contain no 

 chlorophyll, but leaves which are dtiring growth, brown, red, or 

 otherwise highlv colored contain chloroplasts, the green color in 

 such cases being veiled by the presence of other pigments often 

 present in the cell-sap. The existence of variegated leaves of the 

 white type is an abnormal and not a natural phenomenon ; and this 

 feature occupies the same place in the vegetable kingdom as the 

 albino does in the animal ; in both cases— all other things being 

 equal— a weakened constitution is brought about, due in the case of 

 a plant, to the fact that a variegated leaf with its fewer chloroplasts 

 has not the same food producing power as a nonvaricgated one 

 of the same size with its normal number of chloroplasts. 



In general, the function of the leaf as an organ is five-fold : 

 (1) That of carbon dioxide assimilation from the air; (2) that 

 of breaking up this carbon dioxide and the water and its matter 

 in solution sent up by the roots; (3) that of making new sub- 

 stances, or photosynthesis: (4) that of transpiration, and (S) 

 that of respiration ; the latter partly includes the first function. 

 The total work of the leaf is intensely complex and wonder- 

 fully intricate, but it all goes on in that beautifully, orderly 

 and' perfect manner which always characterizes the carrying oiit 

 of Nature's laws, when the environment is as Nature desires it 

 to be. 



Thus the importance of healthy leaves is apparent, but, as 

 before stated, light is necessary for the completion of the manu- 

 facturing processes carried on by them, which explains the why 

 of many facts connected with practical plant growing. In green- 

 houses, however correct the conditions of soil, temperature and 

 moisture, growth is considerably less during short days and 

 periods of little sunshine, than it is in long, bright days under 

 the same conditions otherwise. In northern latitudes with a mid- 

 Summer of almost continuous sunshine, a crop of barley, for in- 

 stance, will be ready for harvest in si.x weeks after sowing, while 

 the same crop growing several degrees farther south may take 

 four months, and so on. 



Both chemical and biological forces appear to be co-operating 

 when the leaf organ carries out its function ; and, like every 

 other phase of energy existing upon earth, what may be called 

 the motive power of this functional activity comes from the sun's 

 light and heat, and the energy, light and heat which was absorbed 

 in this way by the leaves of plants growing upon the earth many 

 thousands or millions of years ago we make use of today from 

 their remains in the form of coal. 



In the leaf the principal seat of chemical action is in the 

 chlorophyll bodies. Chlorophyll is not a simple substance but is 

 compounded of both organic and mineral matter. Iron, phos- 

 phorus and magnesium being the more important mineral elements 

 necessary for its activity. The biological force is active in the 

 protoplasm in which the chlorophyll body is suspended and of 

 which it forms only a small part. Protoplasm is a gelatinous, 

 colorless substance composed chiefly of proteid matter, and in 

 it, that invisible thing known as life exists; in fact protoplasm 

 is the living substance of the plant. No organism, whether plant 

 or animal, can be alive unless its cells contain living protoplasm, 

 and this living protoplasm has come down in an unbroken line 

 from the time ages perhaps ago, when the ancestor of the organism 

 was first created. In other words, protoplasmic life cannot be 

 formed anew, it can only exist today as the result of growth from 

 that which existed in the past. 



When the plant cell is first formed the protoplasm contains no 

 chlorophyll bodies or chloroplasts. Small colorless grains first 

 appear, and then the greening of them takes place. The color- 

 less chloroplast may make its appearance in the absence of light, 



but the last stage of its development is attained only under the 

 influence of light, and requires a higher temperature than is 

 necessary for the first stage of the process. This explains why, 

 that while in cool, dull weather some growth may take place, 

 vegetation looks yellow because there is not suflScient heat 

 and light to fully complete the second part of chlorophyll develop- 

 ment. When plants have a rich, dark, green color, is the time 

 of the largest amount of activity and growth. 



Chlorophyll loses its activity and undergoes a decided change 

 as the plant matures, a phase which is seen more especially in con- 

 nection with annuals and biennials, as well as with those parts 

 of perennial herbs which die down at the end of the season ; the 

 same thing happens also when the leaves of deciduous trees 

 and shrubs lose their green color. The organic contents of the 

 chlorophyll bodies and other cell matter is removed from the 

 leaves and dead stalks, and stored up in the seed, fleshy and 

 other roots and underground buds ; the matter stored being drawn 

 upon the following year when the seed germinates or the plant 

 starts into new growth. In the case of deciduous plants in 

 which the Summer growth other than the leaves does not die, 

 the matter is stored mainly in the buds, flower or otherwise, which 

 are always formed at the end of each Summer ready for com- 

 mencing growth in the Spring. 



A very important practical matter arises in this connection 

 as regards the permanent well-doing and increase of all hardy 

 plants, especially those of a bulbous, or tuberous rooted character. 

 It must be borne in mind that all healthy, thrifty plants manu- 

 facture more food in their cells than they require for their im- 

 mediate growth. If we remove the .green leaves and stems of 

 narcissi, lilies, iris, peonies and such like plants, immediately or 

 soon after flowering is over, the transference of the matter stored 

 in these parts cannot take place, the result being that the flower- 

 buds for the following year cannot be formed, the plant is weak- 

 ened and, if the procedure is continued, the plant will die out. 

 As above mentioned much of the reserve material is transferred 

 to the ripened seed, and, as noted in a previous lesson of this 

 series, no act of plant life is a greater drain upon it than the 

 formation of seed. Therefore, if the production of seed is not 

 required and is not the object for which the plant is grown, the 

 production of it in the case of perennials should not be allowed, 

 which can be accomplished by keeping flower-heads cut oft' 

 immediately the petals drop, a method which also secures to a 

 greater or lesser extent according to species, a more or less 

 continuous production of flowers, when, if seed is allowed to 

 form, only one crop would be produced. Even in the case of 

 those species that never give liiore than one crop of flowers in 

 a season, by removing the flower-heads, the material which would 

 be transferred to seed will be sent down to the roots and root 

 buds for the next season's use, and by this means they are 

 strengthened. 



The chief function of chlorophyll, assisted by the protoplasm, 

 is the formation of starch, from which starch all the matter em- 

 braced in the term carbohydrates is derived. If green plants should 

 stop the manufacture of carbohydrates the food supply of the 

 world would soon be exhausted. All other forms of food are in 

 some way derived, partly or entirely, from starch, and there is 

 no way by which this can be produced except through the leaves 

 of plants. 



The first function of leaves is the obtaining of carbon, which 

 they do in the form of carbon dioxide, from the air. The im- 

 portance of this is understood when we realize that about one- 

 half by weight of the dry matter of both vegetable and animal 

 tissue is carbon and the original source of all this carbon is from 

 the air through the leaves of plants. While the amount of carbon 

 dioxide contained in the air seems almost negligable, the quantity 

 being normally not more than three one-hundredths per cent 

 (.03) although this amount may be temporarily greater in manu- 

 facturing districts and in the neighborhood of large cities, still 

 the quantity in the present atmosphere is sufficient for all the 

 needs of plants throughout imaginable time. Further, while 

 vegetation is continually drawing carbon dioxide out of the air, 

 there is practically an equal amount being returned to it from 

 animal respiration — the quantity estimated for mankind alone is 

 not less than fifty million tons per day — and the consumption of 

 fuel. While the percentage amount in the atmosphere is small 

 the total quantity is quite large, as over each acre of the earth's 

 surface there are about thirty tons of carbon dioxide always 

 at the disposal of plants. 



,As above mentioned, one important operation taking place in 

 the leaf is the manufacture of carbohydrates, of which the prac- 

 tical starting point is starch, and from the latter all the other 

 carbohydrates, such as fat, sugar and cellulose are derived. The 

 raw materials used- by the leaf in the manufacturing process are 

 water which is obtained from the soil by the roots, and the carbon 

 dioxide which enters the leaf through the stomata. Water is 

 (Continued on page 519) 



