438 PHYSIOLOGY 
If a gemma (p. 98), which when separated from the parent is just 
alike on the two sides, be grown in a moist chamber with the lower 
side illuminated and the upper dark, air chambers will be developed 
on the lighted side and rhizoids on the dark one, exactly the reverse 
of the usual relation. Gravity, if it furnish any stimulus, as is prob- 
able, is clearly overcome by light. In like manner light determines 
the formation of the sex organs upon the under side of fern prothallia. 
A striking example of light effects among the seed plants is to be found 
in the dorsiventrality of the rootstocks of the spatter dock (Nymphaea 
advena). These great rhizomes develop at the surface of the mud at 
the bottom of pools, and are of the length and thickness of a man's 
arm. From the upper side numerous leaves arise, and from the under 
side roots. This distribution of organs is found to be determined by 
differences in lighting. 
Electric waves. Of the same class as heat and light waves are the 
electric waves; and they too have considerable formative influence. It 
has been shown that the germination of many seeds is hastened by suit- 
able electric stimuli, and for a considerable time the growth of seedlings 
is also accelerated. When crops of barley, wheat, beets, and other 
economic plants are frequently subjected to a quiet discharge of high- 
tension currents from wires, with many pendent points, strung over the 
experimental fields, it has been found by several observers that the 
plants grow better, come to maturity earlier, show increased productiv- 
ity, and are of better quality than on control plots. 
Thus, an electrified wheat plot of 3 acres yielded a crop 39 per cent greater than 
the control plot, sold at 7.5 per cent higher prices, and the flour was of a higher 
grade on account of it baking quality. Beets (for the table) on an electrified plot 
showed 33 per cent increase and contained an average of 8.8 per cent sugar, against 
7.7 per cent on the control plot. 
Chemical agents. Chemical stimuli are also extremely important 
in determining the form of plants. The presence or absence of particular 
substances in the cells, whether foods or wastes, doubtless exerts a pro- 
found influence. But the precise influence of the different compounds 
cannot be determined satisfactorily, because the chemical processes 
within the plant are so imperfectly known. It is in this region that the 
role of the so-called necessary elements of the ash, calcium, magnesium, 
potassium, and iron are to be sought, in all probability. How far the 
xerophytic structure of plants is to be ascribed to the lack of water is not 
certain. The deficiency of available water may be in itself a chemical 
