32 PLANT STRUCTURE AND GROWTH. [CHApP. Il. 
hood, quinine from the bark of the chinchona, and 
strychnine from the seeds of Mux 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. 
