tlOOTS. 



95 



tern, as if seeking for water and the essential materials. 

 Sometimes the root system becomes remarkably extensive, 

 visiting a large amount of soil in order to procure the 

 necessary supplies. Sometimes the soil is poor in heat, and 

 root activity is interfered with. In such cases it is very 

 common to find the leaves 

 massed against the soil, thus 

 slightly checking the loss of 

 heat. 



Most soil roots also need free 

 air, and when water covers the 

 soil the supply is cut off. In 

 many cases there is some way 

 by which a supply of free air 

 may be brought down into the 

 roots from the parts above 

 water ; sometimes by large air 

 passages in leaves and stems 

 (see Figs. 87, 88, 89, 90) ; some- 

 times by developing special root 

 structures which rise above the 

 water level, as prominently 

 shown by the cypress in the 

 development of knees. These 

 knees are outgrowths from roots 

 beneath the water of the cypress 

 swamp, and rise above the water level, thus reaching the 

 air and aerating the root system (see Fig. 91). It has been 

 shown that if the water rises so high as to flood the knees 

 for any length of time the trees will die, but it does not 

 follow that this is the chief reason for their development. 



65. Water roots. A very different type of root is devel- 

 oped if it is exposed to free water, without any soil relation. 

 If a stem is floating, clusters of whitish thread-like roots 

 usually put out from it and dangle in the water. If the water 

 level sinks so as to bring the tips of these roots to the mucky 



FIG. 90. Longitudinal section 

 through a young quillwort leaf, 

 showing that the four air cham- 

 bers shown in Fig. 89 are not con- 

 tinuous passages, but that there 

 are four vertical rows of promi- 

 nent chambers. The plates of 

 cells separating the chambers in 

 a vertical row very soon become 

 dead and full of air. In addition 

 to the work of aeration these air 

 chambers are very serviceable in 

 enabling the leaves to float when 

 they break off and carry the com- 

 paratively heavy spore cases. 



