32 PLANT STRUCTURE AND GROWTH. [CHAP. n. 



hood, quinine from the bark of the chincliona, and 

 strychnine from the seeds of Nux vomica. Tea and 

 coffee owe their well-known refreshing qualities to 

 the presence of certain alkaloids in their cells. 



The albuminoids are stored up in the protoplasm 

 of active, living cells, and in many seeds in the form 

 of albumen, &c. Other substances in small quanti- 

 ties are also stored up in the cells, as sulphur, iron, 

 potash, silica, lime, phosphorus, magnesia, &c. 



The conditions necessary for the growth of plants 

 are soil, air, moisture, and heat above the freezing- 

 point. The quantity or degree required varies with 

 the species, some requiring great depths of soil in 

 which to develop their roots, others requiring but 

 little, whilst in the case of misletoe and some tropical 

 species of orchids soil is quite unnecessary, the plants 

 growing on the trunks of trees. Some water-plants 

 have the whole surfaces of their leaves and stems 

 submerged, and therefore must be dependent on the 

 water for their carbon. Many ferns and bog-plants 

 require copious supplies of moisture, whilst succulent 

 plants, as the Stonecrops and Houseleeks, delight in 

 dry, stony places. With respect to heat, plants are 

 widely distributed over the whole surface of the 

 globe. The colder regions produce very few species, 

 and those sparingly, but in hot, damp countries vege- 

 tation is very profuse. 



