HYDROPHOBIA. 382 HYDROPHYTES. but it is of the utmost importance to begin the treatment ns soon as possible after the iiijiirj-, as the prospcet nf siu-iess ;;riivs less and less day by day of delay. Folluwiiij; are the results of the anlirabic treatment at the Institut Pas- ,teiir, I'aris. from 188l> through 18'J!t: 21,«31 cases were treated, with !I9 deaths; showing a mortality of 0.45+ per eent. In IdOO there were tliirty-live I'astenr institutes in the world, at the fallowing plhees: Kight in I"' ranee, si. in Rissia, live in Italy, two in .ustria. one in New York City, one in Chicago, one in Baltimore, one in Havana, one in Kio .laneiro. one in Buenos Ayres. one in Saragossa. one in .Malta, one in Bucharest, one in Constantinople, one in .Meppo. one in Till is, one in Algiers, and one in .thens. P.SEriK)K. iE.s. or Lyssopiiohia, is a neuras- thenic or hysterical condition, in which the patient, who inuigincs that he has rabies, in his morbid condition of mind enacts some of the symptoms, and may even be frightened to death. Consult: Youatt. On Ciininc Madness (London, 1830); Suzor, Ih/dioiiliDhiti: An Account of M. I'astcur's Si/slcm ( I^ondon, 1887); Babes, in Annales dc I'Jnslitui I'asteur, vol. v. (Paris, 1892) ; Byron, Kesearches in the hoomis Labora- tory (New York, 1890) ; Pasteur, in Comptca rendtis de I'Acadi^mie den Sciences (Paris, 1881 and 1889), and in Annates dc I'Institut Pasteur (Paris, 1887-88); Salmon. "Development of Knowledge Concerning .Animal Diseases." in V. K. DrpartinrnI of Aur'aiHin'c. Hiiicau of Animal Industry, ,Sixlc< nlli Annual Itrport (Washington, 1899). HYTJROPHYTES (from Gk. vSup, hydOr, water + 0in-di', phylon. plant). Plants which t;row naturally cither in water or in very wet soil. This term is contrasted with mesophytes and xerophytes (qq.v.). Common illustrations of hydrophytic plant societies are swamps of all kinds, poncl .societies, and ocean plants. Various classes of hydrophytes are taken up under sepa- rate heads, where the main features of the va- rious hydrophytic societies will be discussed. It will be desirable, however, to give a short ac- count of the characteristic hydrophytic struc- tures. The roots of hydrophytic plants are in most cases very greatly reduced, and in some cases altogether lost, ns in some of the duck- weeds. Root-hairs are also commonly lacking in water plants. The stems and leaves are not as a whole conspicuously reduced in water plants, but they show peculiarities of .structure that are quite interesting. The leaves of hydro- phytes are frequently liiiely divided, as in the water-milfoil and mermaid-weed. In many cases where the leaves are not finely divided, they are very thin; for example, in tape grass. An ex- amination of the structure of the leaves shows the entire absence of stomata in the submerged parts, complete or almost complete absence of palisade cells, and a very thin ei)idermis which contains chlorophyll. The stems sutler a noteworthy re- duction in the size and development of the water- conducting ves.sels and mechanical tissues, and a great increase in airspaces. The structures just described are found in their highest develop- ment in submerged water plants. Hydrophytes whose leaves are aerial show no conspicuous dif- ferences from ordinary land plants in most re- spects. One elas< of bydropliytes, which may perhaps be called amphibious plants, shows some remarkable variations, especially in structure. I.«ave8 which are developed under the water show the characteristic .structures outlined above, including the tine leaf division, whereas leaves of the .same plant developed in the air show typical aerial leaves witlmut these divisions and with |>alisades, stomata, and a thick cuticle. The stimulus or stinuili which cause these wide varia- tions are not certainly known, but they are dis- cussed to some extent in the article Leaf. Ccmi- mon American plants, which show variations to a high degree, are the aquatic buttercups, the mermaid-weed, some of the crcs.ses, and water- hendocks. However the hydrophytic structures that have been descrilx^d in the i)receding paragraph may liave arisen, certain advantages can be clearly seen, at least in some cases. The thin walls of the epidermis, which are in striking contrast to the thick cutinized walls of many aerial leaves, permit the easy entrance of water and substances dis.solved in the water. On this account many submerged plants are practically independent of soil relations; they take in most of their mate- rial directly from the water. It is easily possible to grow cultures of many of these plants without having any root connection whatsoever. While certain forms, such as the water-milfoil and the water-weed (Elodea), develop roots in ordinary aquarium cultures, other forms, such as horn- wort (Ceratophylluni). never develop roots and yet grow quite as vigorously as in their natural rooted condition. A few forms, such as the hladderwort ( I'tricularia) and some of the duekwiH'ds, have no roots in nature. Of ccnirse, in such cases, the entire absorption of nutriment must take place through the leaf epidermis. The fact that the leaf epidermis contains chlorophyll is also a matter of advantage, since water very soon destroys the elliciency of rays of light. At a comparatively shallow depth there is a cessa- tion of the development of green plants. A re- duction in the water-conducting tissues, while not necessarily an advantage, is, nevertheless, not harmful, inasmuch as the absorption is so largely through the leaf, instead of the root as in land plants. The reduction in root develop- ment is not so easy to understand, since it would seem that holdfast organs would ordinarily be > of advantage; then again, any absorption which the roots might make would so much the more increase the capacity of the plant. A high de- velopment of air-cavities is a distinct advantage, not only to help tloat the plant, but probably to a much higher degree to act as a sort of air- storage. It can readily be seen that the condi- tions for obtaining air underneath the water are not of the best, and that any additional means for obtaining or for storing air would increase the plant's efficiency. The reduction in the de- velopment of mechanical tissues, of palisade cells, and of stomata is not necessarily an advantage to water plants, but since these tissues arc not actually needed, the plant loses nothing by its failure to develop these structures. The peculiar leaf forms that have been noted above are, per- ha])s, not necessarily of any exceptional advan- tage. It must not he supposed that everything in a plant can be explained in accordance with the need of the plant. It is much more likely that the explanations should he referred to defi- nite chemical and physical causes. However, in the case of finely divided leaves, it can be .seen that a much larger proiwrtion of cells comes in
