THE BOOT 39 



the mineral matters in question. These then pass into the 

 root in solution, and are conveyed to the parts of the 

 plant where their presence is required. 



55. As the food sought becomes exhausted the root 

 hairs cease to act, and after a short time die and fall away. 

 Meanwhile further on new hairs have been put forth in 

 soil lately invaded. These likewise serve their turn and 

 shrivel. In this manner the root tip in its progress is 

 followed by a belt of absorptive organs which explore the 

 soil on every side of the line of advance. 



56. Root hairs are the chief organs for the absorption of 

 water and dissolved mineral salts, in the usual cases. 

 They are, however, wanting in many aquatics and even in 

 some terrestrial plants. 



57. Protection of the root 

 tip. — In growth new tissue 

 is formed close to the end of 

 the root (see Fig. 28). The 



very forefront, subject to 28. The end of a growing root, tipped 



wear and tear by the resist- and protected by the root cap ; 



g, the growing point. (Con- 

 ance of the sod to the root S siderably magnified.) 



advance, is furnished with a 



shield of tissue, somewhat in the form of a thimble, which 

 is renewed from the growing point within as fast as it is 

 worn away externally. This is called the root cap. 



58. Aerial roots are such as are produced above ground. 

 Some of the most highly specialized aerial roots are those 

 adapted to tlie absorption of rain and dew. Epiphytes — 

 that is, plants seated upon other plants, but not living at 

 their expense — are obliged to depend upon occasional 

 supplies of water, which the roots take up rapidly at the 

 time and pass on to the leaves and stem to be stored for 

 future use. Epiphytic orchids accomplish this by means 

 of a thick spongy layer covering nearly the entire length of 

 their numerous aerial roots (Fig. 29). 



59- Absorption of organic food. — The waste from decaying vegeta- 

 tion is made use of by a very large number of plants having no other 

 means of support. These are saprophytes. They are mainly Crypto- 



