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 its 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 



