442 PHYSIOLOGY 



shoots be cut away, some of the subterranean shoots will turn up into the air, be- 

 come green, and develop foliage and flowers as though never inclined to be subter- 

 ranean. The sporophylls of certain ferns, notably Onoclea, are entirely different in 

 aspect from the nutritive leaves, and have so many sporangia crowded on the sur- 

 face that they seem entirely covered. If all the nutritive leaves be cut away, leaves 

 that ordinarily would have become sporophylls will then become foliage leaves 

 and bear no sporangia. In like manner the tendrils of the pea leaf may be made to 

 develop into leaflets. 



In all these cases transformation is possible only before the primordia 

 have gone too far in any determined course, though the point at which 

 new influences may affect them is very different in the different cases. 

 Usually the stimulus must be applied very early, while the primordia are 

 still undifferentiated. Many of the problems of regeneration are com- 

 plicated by these phenomena of correlation, if they are not wholly de- 

 termined by them. 



4. NASTIC CURVATURES 



Epinasty and hyponasty. — A somewhat less general manner in which 

 stimuli of various sorts affect plants is to be found in their effects upon the 

 rate of growth on the two faces of bilateral organs, such as thalli, foliage 

 leaves, bud scales, perianth leaves, etc. It is very common to find that 

 such organs grow at different rates on the two faces, so that they are 

 distinctly curved thereby. Thus, in their earliest stages, the leaves 

 grow fastest on the back or outer side, so that the inner face is pressed 

 close to the axis, and as they usually outgrow it, they curve together over 

 it in a protective fashion, forming a bud. The scales, especially, long 

 maintain this form, as the longitudinal section of any bud will show. 

 Later, the relative rate changes; the inner face grows more rapidly than 

 the outer, and the bud opens because the curvature carries the leaf or 

 scale away from the axis. Thalli often show the same thing; the upper 

 surface may be so tense from greater growth that the thallus is tightly 

 appressed to the ground. Such curvatures are described briefly by the 

 terms epinasty or hyponasty, according as the greater growth is on the 

 upper (inner) or lower (outer) face. The greater number of these nastic 

 curvatures are due to unknown (internal?) causes, but some have been 

 found to be reactions to external stimuli (paratonic). The former are 

 not unlike those autonomic curvatures of radial organs described as 

 nutations (p. 423), only in this case the bilateral structure of the organ 

 determines that the nutations shall be in one plane only. The latter 

 are also allied to tropisms, but differ from them in that not the direction 



