306 A TEXT-BOOK OF BOTANY. 



the stem just above the surface of the earth and attaching thereto 

 a glass tube by means of a tightly-fitting rubber tube. It is de- 

 sirable to perform this part of the operation under water and to 

 have the glass tube partly filled with water at the beginning of 

 the experiment. This is done to prevent the clogging up of the 

 vessels with air, which prevents the ready passage of fluids 

 through them. If the root is now kept moist, the osmotic pressure 

 of its cells forces water up into the glass tube, sometimes to a 

 height of several feet. Experiments on the begonia and on many 

 other plants succeed very well, but for some reason the geranium 

 is impracticable to work with. The manometer devised by Ganong, 

 while not showing the quantity of water forced up by the root, 

 shows the amount of pressure exerted, which is really the most 

 important fact to be ascertained. 



Modified Roots. Roots which arise from the nodes of the 

 stem or other parts of the plant are known as secondary or adventi- 

 tious roots. These include the aerial roots of the banyan tree 

 and the Mangrove (Fig. 165), which are for the purpose of sup- 

 port ; the roots of the ivy, which are both for support and climb- 

 ing, and the roots of Indian corn and many palms which serve both 

 for support and the absorption of nourishment. Under this head 

 may also be included the aerial roots of orchids and the root-like 

 structures, or haustoria, of parasites, as of mistletoe and dodder, 

 which penetrate the tissues of their host plants and whose vascular 

 strands come into most intimate relations with those of hosts. 



Of special interest 'also are the breathing roots of certain 

 marsh-plants which serve to convey oxygen to the submerged 

 parts ; and the assimilation roots of certain water-plants and 

 epiphytes, which are unique in that they produce chlorophyll. 

 In certain plants the roots give rise to adventitious shoots, as in 

 Prunus, Rubus, Ailanthus, etc., and in this way these plants some- 

 times form small groves. 



Root Tubercles. The roots of the plants belonging to the 

 Leguminosae are characterized by the production of tubercles, 

 nodules or swellings (Fig. 167) which have been shown to have 

 a direct relation to the assimilation of nitrogen by the plants of 

 this family. Like carbon, nitrogen is one of the elements essential 

 to plant-life, being one of the constituents of protoplasm and 



