Roots. 



41 



has no green leaves, as in the case of dodder (see Fig. 15), 

 it must depend upon its host for all kinds of food mare- 

 rials, — for the starches, sugars, oils, and proteids manufac- 

 tured by its host. It is, in other words, a complete 

 parasite. But if, as in the case of the mistletoe, it has 

 green leaves of its own, it is entirely dependent on its host 

 for the water and dis- 

 solved soil materials 

 only, and is then but 

 partly parasitic. 



31. Roots of Air Plants. 

 — The aerial roots of 

 some tropical orchids 

 and of other aerial plants 

 (see Fig. 16) do not be- 

 come embedded in a sub- 

 stratum, but grow free in 

 the air, and they must, 

 therefore, be able to 

 absorb rapidly the water 

 which falls or gathers on them from the rain or dew. 

 To accomplish this the outer layers of the cells of the 

 roots are empty and their walls are perforated by minute 

 openings through which the water can be drawn by capil- 

 larity. It may be that the water vapor of the atmosphere 

 is condensed within these cells, but experiments on this 

 subject have given contradictory results. 



32. Prop Roots. — The prop roots growing at the basal 

 nodes of Indian corn, and the famous prop roots of the 

 banyan tree, grow downward to or into the soil. Supported 

 in this way, the banyan tree is able to spread its branches 

 over an area so large as to give shelter to an entire 

 village. 



Fig. is. 



A J Cuscuta Europaea, or Dodder, twining about 

 and parasitic on a hop vine and bearing a 

 cluster of small flowers. 



B^ diagrammatic drawing of a cross section of 

 a hop vine through the plane where the roots 

 of the dodder enter it and penetrate to its 

 vascular bundles. X lo. After Kerner, 



