188 PRACTICAL LESSONS IN SCIENCE. 



they become tendrils, spines, and in some cases cups. Besides 

 these there are numerous outgrowths of the epidermal system, 

 known as bristles, prickles, scales, glands, root hairs, etc., which 

 are called trichomes. 



The roots which fix the plants in the ground, or on some sup- 

 port, and absorb nutriment for the plant, show little variation 

 in form. The roots of parasites are usually short and thick, and 

 sometimes provided with sucker-like organs. And the aerial 

 roots of orchids, and similar plants, often have a thickened 

 epidermis containing more or less chlorophyll. 



All parts of a growing plant are abundantly supplied with 

 water; it makes up about 75 per cent, of the weight of plants. 

 Water is constantly being taken up by the roots, and as con- 

 stantly being evaporated from the leaves. It has in solution, 

 materials from the soil and soluble products of plant action. 

 The tissues are constantly taking materials from this liquid for 

 their growth, and the various secretions, as oils, gums, acids, 

 etc., are formed from materials taken from the same source. The 

 cause of the movement of water or sap in plants is not fully 

 understood. 



In the chlorophyll parts of plants during the daylight water 

 and carbon dioxide are broken up into materials for starch, cel- 

 lulose and other hydrocarbons, and oxygen is set free. This pro- 

 cess is called assimilation. These materials are then changed to 

 sugar, cellulose, protoplasm, etc., by a process called metastasis, 

 or metabolism. This process is not understood, but it seems to 

 go on more rapidly at night, and to be attended with the evolu- 

 tion of carbon dioxide, while assimilation is attended by the 

 evolution of oxygen. 



In many plants starch or other material is stored up for the 

 future use of the plant in modified stems, as potatoes, onions, 

 etc. ; in seeds, as in the various grains, and in buds. In most 

 cases this material is used as a supply of food for the growth of 

 young branches or the young plant. Some plants are parasites, 

 taking materials already assimilated from other plants. Again, 

 they are partially parasitic, where some food is assimilated in the 



