UNEQUAL GROWTH. 



767 



These movements of nutation of bilateral appendicular organs take place mostly 

 in one plane which coincides with the median plane of the organ. As long as the 

 organ grows most rapidly on the dorsal side, it may be termed, after de Vries, 

 hyponastic, afterwards, when it grows most rapidly on the inner or upper side, 

 epiuasiic. Since in the later stages of development of an organ growth ceases at 

 certain places — while at different distances from these places it presents different 

 stages of growth, until it finally ceases everywhere — it is clear that in the same organ, 

 by the side of spots where growth is completed and nutation no longer takes place, 

 others occur with hyponastic and others again with epinastic growth, until at length 

 nutation and growth alike cease altogether, as in Fern-leaves. 



Seedlings of Dicotyledons are a remarkable illustration of bilateral structures 

 which nutate in one plane ; although their stem and primary root become afterwards 

 multilateral and grow vertically upwards and downwards. The stem terminates in 

 a pendent or nodding bud ; and the curvature, which is generally very great, exhibits 



FIG. 454. — Nutation of the filaments of Dzctanntus Fraxniella ; the filaments of the stamens whose anthers have not 

 yet opened are bent downwards ; those with anthers already burst are bent upwards. 



itself also in germination when it takes place out of the ground, in a vessel that 

 rotates slowly round a horizontal axis; it is a true curvature of nutation inde- 

 pendent of light and gravitation. But the older portions of the stem become straight 

 as they develope from the curved portion ; and in proportion as the stem increases 

 in length, the straight part which bears the nodding bud also lengthens. When 

 germination takes place in a feeble light, or better in a slowly rotating vessel, a 

 more rapid growth occurs of the older portion of the stem which was at first con- 

 cave, causing it to become convex on this side ; and hence the older and younger 

 parts of the stem form together a letter S, as in Phaseolus, Vi'cia Fada, Polygoniijji 

 Fagopyriwi, Cruciferae, &c. But the primary roots of dicotyledonous seedlings also 

 manifest a tendency to a bilateral structure ; since, when they develope under slow 

 rotation round a horizontal axis, they seldom continue to grow straight, but curve 

 concavely either in front or behind, sometimes even becoming rolled up. These and 

 other instances of nutation are not clearly seen when the development takes place 

 under normal conditions, because the growth of the stem of the seedling is retarded 

 by light, and the curvature both of stem and root prevented by geotropism. 



