In this chapter, an attempt has been made to arrange in some semblance of order some of the known manifestations of alteration of fixed habits of growth of plants in response to changes of environment, and to point out some of the suggestions of possible explanations of these phenomena which have been presented by different investigators. This presentation cannot be considered as anything other than an introduction to a field of study which is as yet almost entirely unexplored, and, like all other unexplored territory, is full of mysteries. If the study of this chapter serves to stimulate interest in these mysteries and wonders of plant life, its purpose will have been accomplished. INDEX Bold-face figures indicate main references Accelerators, 196. Accessory substances, 19. Achroo-dextrin, 61. Acid, acetic, 125, 126, =128=, 132, 133, 166. arabic, 68. arachidic, 133. aspartic, 168, 177. brassic, 133. butyric, 126, 133. capric, 133. caprylic, 133. carnaubic, 140. cerotic, 133, 140. citric, 125, 127. convolvulinic, 81. crotonic, 133. diamino-oxysebacic, 169. diamino-trioxydodecanic, 169. digallic, 96. ellagic, =96=. euxanthic, 84. formic, 25, =126=, 128, 132. galactonic, 42. gallic, =96=. geddic, 69. gluconic, 42. glucuronic, 42, 43. glutamic, 168, =177=. glycero-phosphoric, 142. hydrocyanic, 77. jalapinic, 81. lauric, 133. lignoceric, 133. linoleic, 133. linolenic, 133. malic, 124, =127=. malonic, 124. mannonic, 42. melissic, 133. meta-pectic, 68, =70=. mucic, 68. myristic, 133. nitric, 125. nucleic, =162=. oleic, 133. oxalic, 68, 124, 125, =126=, 128. palmitic, 133, 140. parapectic, 70. pectic, =31=, 70. phosphoric, 141, 142, =162=. propionic, 126, 166. pyrocatechuic, 96. quercitannic, =98=. racemic, 54. ricinoleic, 133. ruberythric, =83=. saccharic, 42, 68. salicylic, 81. sarco-lactic, =128=. stearic, 131, =133=. succinic, =127=, 128. sulfuric, 125. sylvinic, 149. talonic, 42. tannic, 97, =127=. tartaric, =127=. uric, =160=. xanthoproteic, 173. Acid amides, 151. Acidity of protoplasm, =234=. Acid glucosides, 81. Acid potassium oxalate, 125. Acid potassium sulfate, 88. Acid salts, =124=. Acids as toxins, =246=. Acid sodium sulfate, 125. Acrolein, 135. Acrose, 28. Activators, 196. Adamkiewicz's reaction, 173. Adaptations, =249=. Adenase, 190. Adenine, =160=, 162. Adipo-celluloses, =74=. Adsorption, =214=. Æsculetin, 81, 82.