The criticisms and summary of facts on which the hypothesis regarding protoplasm here adopted is based are developed at length in Kassowitz, _Allgemeine Biologie_, Wien, 1899, B. I. and II., where the collected literature may be found, and the reader introduced to the huge mass of controversial writings put forward since Darwin and associated with the names of Weismann and others. It will probably be noticed that I have employed the term molecular unit of protoplasm, and have not discussed the question of organised structure in the latter: this is because it seems clear to me that living protoplasm as such does not possess "organised structure" in the true sense of that term--it is, rather, busy preparing and making "organised structure," and a molecular constitution would have to be ascribed to all "physiological units" of the nature of micellæ, pangens, ids, etc., as truly as to the structural units of a starch-grain or cell-wall, or even of a crystal. In this connection, the student will find the necessary points of view put forward in Pfeffer, _Physiology_, pp. 37-83. INDEX. Absorption by roots, 49. Absorption of energy, 23. Absorption of light, 27. Absorption of water, 50. _Abutilon_, 183. _Acarus_, 88. Accessory buds, 259. _Acer_, 214. Acid gases, 181, 191. Acids, 130, 136. Acquired characters, 283. _Acrostalagmus_, 238. Action of the environment, 271. Adaptation, 176. Adapted races, 177. _Adonis_, 220. Adventitious buds, 224, 225, 257, 260. _Æcidium_, 88, 114, 116, 187, 188, 189, 217, 223, 225, 232, 247, 252. Aeration, 104. Aerobic organisms, 57. Aetiology, 89, 100. _Agaricus melleus_, 115, 143, 145, 234. Agents of disease, 113. _Aglaospora_, 223. Agriculture, 65. Agricultural Chemistry, 2. _Ajuga_, 217. Albinism, 179, 182, 183, 186. Alder, 207, 219. Aleurone layer, 173. Algae, 215. _Allium_, 258. Almond, 168. _Alnus_, 214. _Aloe_, 134, 161. Alpine plants, 250, 279. American blight, 164, 219. American vines, 155, 169, 172. Amides, 31. Amoeba, 144. Amount of energy stored, 25. Amygdalin, 173. _Anabaena_, 128. Anaerobic bacteria, 58, 237. _Anagallis_, 261. Analyses, 65. Analyses of waters, 58. Anemone, 247. Animals, 99, 108, 142, 207. _Antennaria_, 232. _Anthonomos_, 249. Anthrax, 144. Antiseptics, 162. Ants, 232. _Aphis_, 88, 109, 161, 165, 188, 213, 214, 232, 241, 253. _Aphrophora_, 233. Apogamy, 257, 261. _Aporia Crataegi_, 187. Apospory, 257, 261. Apple, 170, 171, 187, 189, 192, 206, 217, 218, 219, 223, 226, 231, 233, 248, 249, 253, 254. Apricot, 188, 206. Apricots, 234. Area of root-surface, 37, 39. _Arisarum_, 188. _Aristolochia_, 259. Aroids, 113. Arrest of growth, 246. Arsenic, 162. Artificial wounds, 194.