THE AMERICAN BOTANIST. 15 



(Papaz'cr orientale) gave a faint yellowish stain to paper, but 

 immediately turned to scarlet when a weak acid was applied. 

 A photographic negative placed on paper of this kind and 

 exposed to the sun for some time, caused the color to fade 

 out, but upon the application of acid, the picture came out 

 in vivid scarlet. All this is concerned in some way with the 

 familiar fact that a red geranium may be turned from red to 

 blue and back again by the proper application of acids and 

 alkalies. 



Plant Stimuli. — There are many things that afifect the 

 direction of growth in plants. We are most familiar with the 

 response of the plant to gravity seen in the seedling, whose 

 first root invariably travels in the direction of the pull of 

 gravity, while the shoot grows against this force. As in 

 most departments of botany, there are technical terms for the 

 response to each stimulus, and thus we have thermotropism — 

 a turning toward heat ; heliotropism — a turning toward the 

 sun; phototropism — a turning toward light; hydrotropism — 

 a turning toward moisture ; geotropism — a turning toward the 

 earth, and thigmotropism — a turning caused by contact as in 

 the tendrils of various climbers. 



Luminous Plants. — The daughter of Linnaeus is 

 credited with the discovery that certain flowers emit rays of 

 light under favorable circumstances, but it is to the flowerless 

 not the flowering plants that we must look for the greatest 

 production of light. Most people are familiar with the curious 

 glow that comes upon decaying wood at times. It is com- 

 monly known as fox-fire and was for a long time thought 

 to be produced by the wood itself. Further investigation by 

 German botanists have shown that the luminosity of decaying 

 wood, as well as that of decaying fish and meats, is due to 

 the presence of fungi, principally bacteria, though the under- 

 ground part of a mushroom (Agaricits melleus) also emits 



