12 



THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



tip downward, no matter how it be placed. Mois- 

 ture attracts the root very strongly ; roots have 

 been found in cisterns as much as 200 to 300 feet 

 from a tree. 

 There are two principal kinds of roots, one of 



Fig. 25. Four steps in process of cell-division. Mother-cell at 

 left, far advanced in divison; daughter-cell at right. 



which, the tap-root (Fig. 26), goes deep into the 

 soil, growing straight down and sending out lateral 

 roots at intervals. The other spreads out near 

 the surface of the soil (Fig. 27) and consists of a 

 mass of fine rootlets. It has the advantage of estab- 

 lishing itself quickly and absorbing moisture vigor- 

 ously from the start, thus inducing a rapid growth 

 of the plant. But it cannot utilize the deeper soil 

 food nor withstand drought. On the other hand, 

 tap-roots many endure long periods of drought: the 

 long-rooted Peruvian cotton is said to survive a 

 rainless period of six years. 



A well-developed root system forms a mass of 

 finely interlacing filaments that thoroughly ex- 

 plore the soil. The total length of these has been 

 estimated at a quarter of a mile for a vigorous 

 corn plant, while measurements on a squash vine 

 proved the root_to be over fifteen miles in length. 



Fig. 26. Tap-root. Dandelion. 



the greater part of this being produced at the rate 

 of a thousand feet per day. 



Because of need of air, most roots are unable to 

 thrive in wet soil, and their best work is done in 



soil in which the water is held in a thin film around 

 the soil-particles. Each particle constitutes a 

 minute water reservoir. To reach and tap these 

 reservoirs is the work of the root-hairs, which ap- 

 pear just back of the root-tip as outgrowths from 

 the surface cells of the root (Pigs. 28 and 29). 

 They force themselves energetically between the 

 soil-particles and attach themselves so closely 

 that they often break off rather than loosen their 

 hold when the root is pulled up. Thus they come 

 into contact with the water-films that surround 

 the particles, and by means 

 of water - attracting sub- 

 stances within the root-hair 

 they pull the water away 

 from the particles. As each 

 tiny reservoir is emptied of 

 its supply, water flows in 

 from surrounding ones and 

 these also yield up their 

 stores. 



The water passes from 

 the root-hair through the 

 soft outer tissue (cortex) to 

 the wood-cells, in which it 

 passes directly to the leaves. 

 These ihick-walled wood- 

 cells form groups that al- 

 ternate with groups of thin- 

 walled tissue or bast which 

 conveys proteids and other 

 food from the leaves to the 

 root and to other parts of 

 the plant. The wood and 

 bast are surrounded by a 

 row of small cells (endoder- 

 mis), whose closely joined Fig 

 walls prevent the entrance 

 of air, which would im- 

 pede the progress of water in the wood-cells. 



The absorptive surface of the root may be in- 

 creased from seven to seventy-five times by the root- 

 hairs. The fine roots, on which the root-hairs are 

 principally produced, are known as " feeding roots," 

 and all tillage should be practiced with special 

 reference to them. Tillage aids the work of the 

 root by increasing the air and water-supply, and 

 by loosening the soil. Roots will penetrate hard 

 soil, or even hard substances like sealing-wax, but 

 they grow very slowly under such conditions. They 

 may develop a pressure of 50 to 100 pounds per 

 square inch. The root-cap protects the delicate tip 

 as it is forced into the soil. 



The water absorbed by the root contains mineral 

 substances. If the plant is burned, these will 

 remain in the form of ashes. By growing plants 

 in distilled water, to which has been added chemi- 

 cally pure salts in various combinations, it has 

 been found that certain substances are indis- 

 pensable to the plant while others are not. The 

 indispensable substances comprise four bases and 

 four acids. The bases are potash, lime, magnesium 

 and iron; the acids are nitric, phosphoric, sul- 

 furic and carbonic — the carbonic acid absorbed from 

 the air by the leaf. If all these substances, with the 



. 27. Fibrous roots. 

 Maize. Aerial or brace 

 roots are shown at o o. 



