376 ADAPTIVE VARIATIONS. 



of, external mechanical forces by the formation of sup- 

 portive tissues. It is by reason of this power that 

 plants grow vertically upwards in opposition to the 

 force of gravity. 



Some of the most remarkable instances of adaptation 

 in plants are those relating to the interchange between 

 roots and stems. In many cases, at least, it would 

 seem that when a subterranean root becomes aerial, its 

 characters tend to approach to those of a stem, whilst 

 a normally aerial stem, grown underground, develops 

 the characters of a root. For example, " An old acacia 

 with a decaying trunk sent down an aerial root from 

 the living part, about six feet from the ground. When 

 it had been rooted in the soil for some time, it became 

 detached by the wind; the root then became a ' stem,' 

 the upper part putting out foliage." * Again, Dr. 

 Lindley records that " a young willow tree had its 

 crown bent down to the ground; this was covered with 

 earth, and soon emitted an abundance of roots. The 

 true roots were then removed from the soil, and the 

 stem inverted. The roots now became branches and 

 emitted an abundance of buds, and the tree ever after- 

 wards grew upside down." Accompanying such 

 changes of function are found corresponding changes 

 of histological structure. Costantin t determined the 

 effect of growing stems of brambles underground, and 

 he found that the number and volume of the cortical 

 cells increased, the collenchyma disappeared, the liber 

 fibres diminished or disappeared, and starch could be 

 formed and stored up in the parenchymatous tissues. 



* Quoted from Henslow, ibid., p. 179. 



f Bull, de la Soc. Bot. de Fr., p. 230, 1883. 



