NATURE OF PLANTS 59 



The cells of the root tip also react to light and heat and also 

 to moisture and food. Roots avoid light and cold and they 

 are thus brought into soils at the proper temperatures. In arctic 

 regions where often only a few inches of the surface of the soil 

 thaws, the roots will be directed straight down to within a short 

 distance of the frozen earth when they will curve at a sharp 

 angle. Every one is familiar with the fact that roots are caused 

 to curve towards and follow the course of decaying tree trunks, 

 being stimulated by the moisture and food contained in them. 

 Numerous instances might be cited where roots are attracted to 

 wells, drains, etc., even surmounting considerable obstructions 

 in order to reach the water. This localization of the sensitive- 

 ness to all external conditions that affect the root at the very 

 tip is altogether admirable, 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 



