KOOTS 



95 



tern, as if seeking for -n-ater and tlie essential materials. 

 Sometimes the root system Ijeeomes remarkal.>ly extensive, 

 visiting a large amount of soil in order to iirocuro the 

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

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

 common to find the leaves 

 massed against the soil, tliiis 

 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 supplj- of free air 

 may lie brought down into the 

 roots from the parts above 

 water ; sometimes by large air 

 passages in leaves and stems 

 (see Figs. H7. 88. 80, 00) ; some- 

 times by developing sjiecial root 

 structures which rise above the 

 water level, as iirominently 

 shown Ijy the cypi-ess 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. 01). It has been 

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

 for any lengtli 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 



Fil;. 90. Lnngitmlinal SL-ction 

 throuizh il yonn^f qiiillwort k'af, 

 showing that the four air cham- 

 Ijers shown in Fig. 80 are not con- 

 tinuous passages, luit that tlu're 

 are four \erlica] ro\\s of promi- 

 nent chambers. Tlie 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 tloat when 

 they break off and carry the com- 

 paratively heavy spore cases. 



