84 DIRECTION OF GROWTH OF ROOTS. 



extremities become true spongioles. When they begin to absorb 



PIG. 36. PANDANUS OR SCREW PINK, a, b, c, aerial roots partly serving as stems; 

 U, e, roots not yet reaching the ground. 



nourishment from the earth, they increase greatly in diameter, 

 and seem like so many assistant-stems. 



111. The general fact is, that the Root is the portion of the 

 axis, which has a tendency to grow downwards towards moisture, 

 and away from light ; whilst the Stem is the portion, which tends 

 to grow upwards, into the dry air, and towards light. This ten- 

 dency is manifested, during the earliest period of the growth of 

 a plant from seed. Two parts always originate from it ; one of 

 which, termed the plumula (from its resemblance, when just un- 

 folding, to a little feather), is the rudiment of the stem and 

 leaves ; whilst the other, called the radicle, is the young root. 

 The first of these exhibits from the commencement a tendency to 

 grow upwards, and ths second a similar tendency to descend. 

 The late ingenious experimenter, Mr. Knight, devised a means 

 4)f showing, that the direction of the roots is in part owing to the 



