64 ROOT FUNCTIONS 



mirable, because it enables it at the very start to come into the 

 most helpful relations to its surroundings. It was a very clever 

 fancy of Darwin to compare this localization to a brain center 

 as found in certain low orders of animals. 



27. Other Root Relations. — While 70 per cent, of our plants 

 develop roots adapted to absorbing substances from the soil 

 there are several interesting modifications of the structures noted 

 above. The roots of aquatics may not develop root hairs be- 

 cause the roots are so finely divided and delicate as to enable 

 them to absorb their crude materials directly from the water. 

 Many plants develop roots in the air. These are sometimes 

 simply clinging devices, as in the poison ivy; or they may be true 

 absorbing organs, in some cases reaching down to the ground 

 or to cup-shaped leaves that hold water, as in many tropical 

 climbing plants. One of the most interesting of these modifi- 

 cations is found in the orchids (Fig. 291) that live on the damp 

 tree trunks or on dripping rocks of tropical countries. Here the 

 root is covered by a thick mantle of cells that are capable of hold- 

 ing the water that comes to them either in the form of dew or 

 drainage or rain. Such plants, called epiphytes, are able to 

 flourish in the air without the assistance of soil roots. Quite a 

 large number of plants, parasites, depend entirely, or in part, upon 

 other plants for their foods. The mistletoe is an example of this. 

 The sticky seeds are carried by birds to the branches of oaks and 

 other trees where they germinate, the roots penetrating the 

 branches and withdrawing the necessary foods. The mistletoe 

 withdraws largely crude materials from the branch because it 

 has pale green leaves and is therefore capable of manufacturing 

 some food on its own account. Our common dodder {Cuscuta) 

 is entirely dependent upon other plants and illustrates the results 

 that follow from the disuse of any function. It has lost its chloro- 

 plasts, the leaves are reduced to inconspicuous scales and the stem 

 resembles a coarse yellow string while the root has disappeared 

 entirely. The seeds produce a small thread-like shoot which soon 

 perishes unless it comes in contact with a plant upon which it 

 can live. These thread-like plants are sensitive to touch and so 

 coil about the branches of any plant that they may chance to hit 



