28 PB ACTIO AL GUIDE TO GABDEN PLANTS 



Root-pressure. — It is well known — indeed obvious — that the tallest 

 tree in the world has water drafted to its highest point. This is an 

 extraordinary fact when it is remembered that the G-iant Sequoia of 

 California and the Gum Trees of Australia often reach the great height of 

 400 to 500 ft. The tips of branches being naturally the youngest and 

 tenderest parts of a plant are always well supplied with water. How 

 the water gets to the outermost tips is a debatable matter and many 

 theories have been suggested ; but the fact remains that it gets there, 

 and the propelling force has been termed ' root-pressure.' It is sup- 

 posed that the water from the fully charged lower cells diffuses or 

 filters through the partitions or walls into the upper cells by a process 

 Called ' osmosis,' referred to above under the chapter on ' Growth.' It 

 can hardly, however, be a mere physical process, as each living cell in a 

 plant contains the living substance called protoplasm already referred 

 to. As each cell containing protoplasm is more or less supplied with 

 watery juice from the roots, it is evident that a vast quantity of moisture 

 is thereby taken from the soil, and passes from cell to cell by root- 

 pressure. "When trees and shrubs are cut in spring during the period 

 of great absorption, large quantities of water are sometimes forced from 

 the cut surfaces, and constitute what gardeners call ' bleeding.' This, 

 if carried to excess, would be injurious, if not fatal, to the plant ; but 

 the surplus overflow as a rule soon becomes checked by the development 

 of fresh leaves, the cells of which must be suppUed vnth sap, and serve 

 to stop the overflow from any wounds. 



Kinds of Roots. ^ — Roots, while all performing the same functions, 

 differ a good deal in appearance. Broadly speaking they may be 

 classified as fibrous, fleshy, and tuberous. All grasses and many annuals, 

 biennials, and herbaceous plants have fibrous roots, while the Dahlia 

 may be taken as an example of a plant with fleshy roots. What are 

 termed ' adventitious ' or accidental roots are those developed from any 

 part of the plant except the seed. The roots of cuttings, layers, stems, 

 &c. are therefore all adventitious. In the case of Monocotyledonous 

 plants (see p. 127) the primary root from the seed soon ceases to grow, 

 and all the roots afterwards developed are from the stem, and are 

 hence mostly adventitious. Their functions, however, are precisely the 

 same as those of the true roots developed directly from the seedling. 



The tubers of Potatoes and Jerusalem Artichokes, the rhizomes or 

 rootstocks of Irises, the corms of Crocuses and Gladioli, and the bulbs 

 of Onions and Lilies, although all naturally growing in the soil are 

 not really roots at all but modified stems, and are referred to below. 



