366 LECTURE XV. 



We have now considered spontaneous irregularities of 

 growth as expressed in nutation, but we have by no means 

 exhausted the subject. Other phenomena belonging to the 

 same category remain to be considered. It has long been 

 observed that in bilaterally symmetrical organs the growth of 

 one surface of the organ is, for a considerable time, more 

 active than that of the other. This is most marked in those 

 bilateral organs which are also dorsiventral, that is, organs in 

 which the two surfaces have a different structure and are 

 endowed with different properties. Thus, in ordinary foliage- 

 leaves, which are characteristically dorsiventral organs, the 

 growth of the dorsal (usually the lower) surface of the leaf 

 exceeds at first and for a considerable time that of the ventral 

 (upper), so that the leaf is more or less folded up upon itself. 

 It is in consequence of this that the young leaves, when they 

 are developed near together at the apex of a shoot, close up to 

 form a bud; and it is also to this that the circinate vernation of 

 many leaves, those of Ferns for example, and the coiling of 

 the young tendrils of the Cucurbitaceae, is due. It is only at 

 a relatively late period that the growth of the other surface 

 becomes the more active. De Vries has introduced a con- 

 venient terminology for expressing these relations. When 

 the dorsal surface is the one which is growing the more 

 actively, the organ is said to be in a state of hyponasty ; when 

 the ventral, the organ is said to be in a state of epinasty. 



These phenomena are clearly allied to the nutations which 

 we have already studied. The cause is in both cases the same, 

 namely, the unequal rate of growth of opposite sides of an 

 organ. The difference is this, that whereas in nutation the 

 relations of the two opposite sides are frequently reversed in the 

 course of the growth of the organ, so that the period of a com- 

 plete cycle is short, in hyponastic and epinastic organs the 

 reversal of the relations between the two sides takes place 

 only once (foliage-leaves) or at most two or three times (sta- 

 mens for instance) in the course of the growth of the organ, so 

 that the period of a complete cycle is very long. Hyponasty 

 and epinasty are simply words used to describe a very slow 

 form of nutation. 



