THE PLANT: ITS STRUCTURE, LIFE -PROCESSES AND ENVIRONMENT 



21 



why a suitable rotation of crops is necessary to 

 preserve the productiveness of the soil. 



The physical condition of the soil is just as im- 

 portant as the chemical. It is almost useless to 

 apply fertilizers to poorly tilled land. The food 

 supply of the soil can be unlocked and made avail- 

 able to the plant only by judicious tillage. 



Heat. — As previously stated, the most favorable 

 temperature for the growth of plants is about 30° 

 Centigrade (86° Pahr.). If the temperature rises 

 much above this point, growth stops, and if the rise 

 continues, death ensues. On the other hand, if the 

 temperature is lowered, growth ceases before the 

 freezing point is reached. Some plants may be 

 frozen with impunity provided they are allowed to 

 thaw out slowly. Others are invariably killed by 

 freezing. 



Too great cold and too great heat have much 

 the same effect on the plant as lack of water. The 

 former prevents absorption by the roots ; the 

 latter causes water to evaporate from the leaves 

 faster than it can be supplied. The habit of drop- 

 ping their leaves on the approach of cold weather, 

 which deciduous trees have, is therefore compara- 

 ble to the action of desert plants in reducing their 

 leaf surface. 



In general, the plant that contains least water 

 is most resistant to heat and cold. Dry seeds have 

 been kept for a long time at the temperature of 

 liquid hydrogen (—238° C, or —396° P.) ; when 

 thawed they grew normally. Bacteria are much 

 m'Ore quickly killed by moist than by dry heat. 

 Frost does not injure buds in winter when they are 

 comparatively dry ; but in spring, when they are 

 full of sap, it quickly kills them. The injurious 

 action of frost is supposed to be largely due to the 

 extraction of water from the cells by the forma- 

 tion of ice in the intercellular spaces. The air that 

 normally occupies these spaces is thereby driven 

 out, so that a frozen leaf, on thawing, resembles 

 one in which the air of the intercellular spaces has 

 been driven out by boiling. It is supposed that 

 when the leaf is thawed slowly enough, the water 

 is taken up again by the cells ; but when it is 

 thawed quickly, the water escapes by evaporation 

 before it can be reabsorbed. 



The action of frost may result in long splits in 

 the trunks of trees, or in the killing of the ends 

 of the branches, which soon blacken in conse- 

 quence. The injured parts should be removed by 

 pruning. 



Air. — Every living cell must have a constant 

 supply of oxygen in order to exist. The stem some- 

 times suffers by applications of tar which shuts out 

 air. The roots commonly suffer and often are 

 killed by being deprived of air. This happens when 

 the soil is too wet or when a hard crust is allowed 

 to form on the surface. For the same reason, 

 paving sidewalks or covering the roots deeply with- 

 soil may be injurious. When the surface of the soil 

 is loose and sufficiently dry, a circulation of air 

 is kept up within the soil by constant changes in 

 barometric pressure. When this is prevented the 

 soil becomes sour and unfit for plants, and the 

 chemical processes that make food available to the 



plant are checked. Roots may grow in running 

 water, which constantly renews the supply of dis- 

 solved air. Some roots can live in mud, but they 

 are supplied with air by way of the leaves and 

 large air-passages in the stem ; they are specially 

 adapted to such environment. It is therefore of the 

 utmost importance to maintain a loose, open tex- 

 ture of the soil by proper tillage, to ensure the 

 health and vigor of most agricultural plants. 



Wind. — The curiously gnarled and bent appear- 

 ance of trees that are daily exposed to strong 

 winds is familiar to all. In many cases all the 

 branches on the windward side are killed. This is 

 due to the drying effect of the wind, which may 

 increase evaporation as much as twenty-fold. The 

 mere mechanical effect of strong prevailing winds 

 is often very marked. It is common to see trees 

 with the tips of the branches permanently turned 

 leeward, or with the heavy growth all on one side. 

 Trees on mountain tops and near sea-coasts are 

 often weirdly picturesque, from wind action. 



The effect of wind in drying fruit blossoms is 

 well known, as well as the mechanical damage to 

 branches laden with ice and snow. For this reason 

 the planting of windbreaks is often indispensable. 



Environment and inheritance. — The facts just 

 mentioned show how readily the plant responds to 

 the influence of environment by altering its struc- 

 ture or functions. The way in which it responds 

 is determined in each case by the qualities it has 

 received from its ancestors. The form of the plant, 

 therefore, depends on both these factors. 



Some plants are plastic and easily modified by 

 external influences ; others are not so readily 

 affected. The very remarkable alterations pro- 

 duced by insects, including the various kinds of 

 galls, the "witches' brooms" produced by attacks 

 of fungi, completely altering the habit of the 

 plant, and the " green flowers" due to small insects, 

 make us realize the great possibilities of external 

 influences. The analysis of all these phenomena 

 should enable us eventually to control them. 



Literature. 



The reader is referred to the following publica- 

 tions for further information : Lectures on the 

 Physiology of Plants, J. Sachs ; the two books. 

 Power of Movement in Plants, and The Various 

 Contrivances by Which Orchids are Fertilized by 

 Insects, Charles Darwin ; Text-book of Botany, E. 

 Strasburger and others ; The Physiology of Plants, 

 W. Pfeffer ; Lectures on the Physiology of Plants, 

 S. H. Vines ; Plant Geography, A. F. W. Schimper ; 

 Organography of Plants, K. Goebel ; Comparative 

 Anatomy of the Vegetable Organs of the Phanero- 

 gams and Ferns, A. de Bary ; Plant Physiology, 

 Paul Sorauer ; Practical Text-book of Plant Phys- 

 iology, D. T. MacDougal ; An Introduction to 

 Vegetable Physiology, J. R. Green ; Text-book of 

 Plant Physiology, G. J. Pierce; the two books. 

 Disease in Plants, and The Oak, H. M. Ward ; 

 Natural History of Plants, Anton Kerner ; The 

 Great World's Farm, S. Gaye ; The Soil, F. H. King. 

 There are many good school and college texts that 

 will aid the general reader. 



