CHAMBERS'S INFORMATION FOR THE PEOPLE. 



combine with the store of nutriment laid up in the 

 seed, and to fit it for the purposes of vegetation. 

 The first organ which ex- 

 pands in the embryo of a 

 young plant is the radicle, 

 or root c. There is a small 

 opening (foramen) in the 

 covering of the seed, to- 

 wards which the point of 

 the root is always turned, 

 in order that it may be 

 protruded without injuring 

 its soft and delicate tex- 

 ture. The radicle takes 

 up water and air, and 

 transmits the liquid thus 

 formed to the other tis- 

 sues. The nutritive sub- 

 stances laid up in the 

 cotyledons, or seed-lobes (a a), become quite 

 changed during the process of germination. The 

 starch, which is insoluble in water, is rendered 

 soluble by the action of a peculiar substance called 

 diastase, derived from the gluten, which acts as a 

 ferment. This substance has so powerful an effect 

 upon the starch as to render it instantly soluble, 

 and thus nutriment is prepared for the use of the 

 infant plant The starch is changed into sugar ; 

 and when the store of starch and gluten has been 

 exhausted, the plant is able to subsist by its own 

 assimilating powers, at the expense of the air and 

 the soil. 



Heat, though essential to germination, is injuri- 

 ous, unless it be combined with moisture. A high 

 degree of dry heat will parch seeds, and destroy 

 their vitality ; hence, when they are to be kept for 

 food, it is not unusual to dry them in an oven, to 

 prevent them from germinating. When combined 

 with moisture, a very high temperature is not inju- 

 rious to many kinds of plants, especially those of 

 low organisation ; for example, various species of 

 plants inhabit the hot springs of Italy and the hot- 

 water pools around the geysers in Iceland ; while, 

 on the other hand, certain lichens and mosses grow 

 in the region of perpetual snow, where the tem- 

 perature seldom rises to 32 Fahrenheit. Warmth 

 is not only necessary for the germination of the 

 seed, but also for the growth and after-development 

 of the plant. In flowering plants the sap will not 

 rise without a certain degree of heat. Cold will 

 also check the development of the flowers and 

 fruit, and even of the leaves, and will prevent the 

 full flavour being attained by the fruit. The secre- 

 tions of plants are diminished by cold. 



Moisture must be combined with heat and air to 

 render it useful to vegetation. An excess of mois- 

 ture without heat, and combined with air, induces 

 decay in seeds, instead of exciting them to germi- 

 nate ; and an excess of moisture is injurious even 

 to growing plants, as it destroys the delicate tissue 

 of the root spongioles. 



Air is essential both to the germination of the 

 seed and the development of the plant. Without 

 oxygen from the atmosphere, the carbon laid up in 

 the seed cannot be made available for the use of 

 the infant plant, as carbon in its concentrated state 

 is insoluble in water, and requires to be combined 

 with oxygen to convert it into carbonic acid gas. 

 In like manner, air is essential through all the pro- 

 cesses of vegetation ; no wood can be formed, no 

 seed ripened, and no secretions produced, without 



66 



abundance of carbon ; and this cannot enter the 

 plant, even from the soil, without a constant supply 

 of oxygen from the air. 



Light is not required for the germination of 

 seeds, but it is essential to the development of 

 plants, as it occasions the decomposition of the 

 carbonic acid contained in the parts exposed to its 

 influence ; without which the plant could not assim- 

 ilate the carbon. Colour also appears to depend 

 partly on light. Plants grown in darkness are of 

 a sickly aspect, and are said to be etiolated, or 

 blanched. 



In addition to the elements of which plants are 

 principally composed, there is also found in their 

 substance a quantity of inorganic matter, which 

 differs according to the nature of the plant, and 

 which appears to be derived solely from the soil. 

 Plants require different kinds of inorganic food, 

 according to their nature, and appear to possess 

 the power of selection, as they only take the kind 

 they need, though it may form but a very small 

 portion of the soil in which they grow. Thus it 

 is evident that any particular crop must in time 

 exhaust the soil in which it grows of the requisite 

 inorganic matters, unless they should be renewed 

 by the addition of what are called mineral manures 

 (see AGRICULTURE) ; and it is also clear that 

 crops requiring another kind of earth may succeed 

 in the same soil after it has become unproductive 

 for the first kind of crop. Hence the neces- 

 sity for what is called the rotation of crops 

 that is, for letting crops of a different nature 

 succeed each other in fields and gardens. Wheat, 

 barley, rye, and oats are silica plants ; pease, beans, 

 and clover, lime plants ; and turnips and potatoes, 

 potash plants. Thus, these crops, from the differ- 

 ence in. their predominant inorganic ingredients, 

 are generally made to alternate with each other. 

 In some soils containing a large amount of phos- 

 phates and other organic matters, the same plants 

 may be cultivated successfully for a number of 

 years. 



Term of Vegetable Existence. The longevity of 

 plants differs according to their nature and the 

 circumstances in which they are placed : thus, 

 plants are annuals, which grow only one season, 

 and die as soon as they have ripened their seed 

 biennials, which generally last for two years or 

 perennials, which last for several years. Trees 

 and shrubs, which have ligneous or woody stems, 

 are destined to remain undecayed for years. The 

 term shrub is applied to those woody plants which 

 branch out from near the root, and seldom attain 

 a great height ; while trees have, generally speak- 

 ing, only one stem or trunk proceeding from the 

 root to a considerable height before it divides into 

 branches. The length of time which trees live 

 depends in a great measure on the situations in 

 which they grow. The age of trees was formerly 

 calculated by their diameter, or by the number 

 of concentric circles or layers in the trunk ; 

 but both these modes are now found to be often 

 fallacious. According to the first, it was sup- 

 posed that if a tree attained the diameter of a 

 foot in fifty years, fifty years should be counted 

 for every foot it measured in diameter ; and thus 

 it was supposed that the great baobab tree, found 

 by Adanson on the banks of the Senegal, which 

 measured nearly thirty feet in diameter, must 

 have been about 6000 years old, or coeval with 

 the world itself. It is found, however, that the 



