55 5 Cultivation of Plants. 



bat-eons tissue. The cells themselves are composed of carbon, 

 oxygen, and hydrogen, and their contents of the same elements, 

 with the addition of nitrogen. With these essential elements 

 several others are associated in different' plants. The com- 

 moner ones are phosphorus, sulphur, silex, potash, soda, and 

 lime. The chemical compounds of organic origin are ternary, 

 quaternary, or even more complicated ; whereas inorganic com- 

 pounds generally are binary. Sugar and starch may be men- 

 tioned as the most familiar vegetable compounds. Amongst 

 vegetable acids, citric, malic, and oxalic are the commonest. 

 Quinine, cinchonine, and morphine are valuable alkaloid drugs. 



The principal phenomena of plant-life coming under our con- 

 sideration are : germination, absolution, and respiration. 



1. Germination. This is the first phase in the develop- 

 ment of independent life in a plant from a seed. In order to 

 accomplish this stage certain conditions are indispensable. 

 These essential conditions are : warmth, moisture, and air. The 

 temperature at which seeds will germinate varies considerably 

 in different species, ranging mainly from 40 to 75 Fahrenheit. 

 But the seeds of some hardy plants will vegetate at a lower 

 temperature, whilst a few tropical things require a still higher 

 degree of warmth to start them into life. There must be 

 sufficient moisture within reach of the seed to enable it to 

 burst its coat by absorption and feed the young embryo. And 

 the access of air is indispensable to effect the chemical changes 

 to which the contents of the seed are subject in germination for 

 the use of the young plant. Unless these three conditions are 

 united in their proper degrees, the seeds will soon perish, 

 especially if there be an excess of humidity. In the absence of 

 moisture, and when not exposed to deleterious atmospheric 

 or other influences, some seeds will retain their germinating 

 powers for many years, whilst others will not grow after the 

 first season. Most seeds contain the nourishment required 

 for the support of the young plant in its earliest stage. This 

 is stored up either in the embryo itself, and chiefly in the 

 cotyledons, or it is present in the form of starch and other 

 ingredients, in the albumen, surrounding the embryo, and 

 constituting in many cases the bulk of the seed. When a seed 

 is committed to the soil, it more or less rapidly absorbs suffi- 

 cient water to soften its coats and distend the tissue of the 

 embryo, causing it to push forth its radicle or rootlet, which 

 invariably turns downwards, no matter what the position of the 



