58 found that exposure to light caused certain flowers to open, and to darkness to close. Probably this is accompanied, as in the parallel case of temperature, by an acceleration of the mean rate of growth. The relation of the long axis of a growing organ to the line of action of gravity appears also to affect its rate of growth. Klfving has found that the sporangiferous hyphaj of Pkycomyccs, which normally grow vertically upwards, grow somewhat less rapidly when they are maintained in the inverse position, that their growth in length is more rapid in opposition to the action of gravity than in the same direction, and this he thinks is true of all organs which normally grow vertically upwards. Gravity would appear, there fore, to exercise a tonic influence on growth. The substitution for gravity of a considerable centrifugal force produced no apparent effect on the rate of growth. Direction The direction of growth of an organ is determined partly by in- of herent and partly by external causes. Beginning with the inlierent growth, causes, we find that, when the action of external directive influences upon growing organs is as far as possible eliminated, the axis of growth that is, the line joining the apex and the base is approxi mately a straight line. Thus Vuehting has shown that, when growing shoots are caused to grow in darkness and to rotate slowly about a horizontal axis by means of an instrument termed a " clino- Inherenfr stat," their long axes become straight. This is the expression of causes, an inherent tendency which he terms " rectipetality." But the line of growth is not at all times straight ; for instance, the apex of an organ growing vertically upwards does not travel upwards in a straight line, but oscillates from side to side of the vertical. This oscillation is termed " nutation," and is due to the fact that growth in length is not uniformly rapid on all sides of the growing organ, but that during any given period of time one side grows more rapidly than the others. This unequal growth, which we may term " heterauxesis, " is apparently spontaneous. The particular path which the apex of an organ describes in the course of its growth depends upon the properties of the organ, a point which may now be conveniently dealt with. Proper- Some information as to the properties of an organ may be ob- ties of tained from observation of its anatomical structure. For instance, growing cylindrical organs, such as many shoots, are radially symmetrical organs, in structure, and they are in most cases found to be also physio logically radial. Some organs, again, such as the leaves of some Irises, are bilaterally symmetrical in structure, and they are found to be physiologically bilateral. Other organs, finally, such as flattened expanded leaves, prothallia of Ferns, thalloid shoots of Liverworts, though, like the bilateral organs, they present two opposed surfaces, are not bilaterally symmetrical in structure, inas much as the tissues in relation with the two surfaces are differently constituted. Such organs are dorsiventral both anatomically and physiologically. The properties of an organ are not, however, always to be inferred from structure. In some cases radial sym metry of structure is accompanied by dorsiventral properties. This is the case, as Sachs has shown, with the shoots of Tropseolum majus under certain circumstances. The question now naturally arises, How are the peculiar pro perties induced ? In some cases they can only be accounted for by regarding them as inherent ; this is true of radial organs, of bi lateral organs, and to a certain extent of dorsiventral organs. The lateral branches of dicotyledonous trees have, in many cases, been found to possess inlierent dorsiventral properties. In other cases, however, dorsiventrality is induced. It has been found by Frank and his observation has been corroborated by De Vries that the dorsiventrality of the lateral shoots of Conifers is induced by their relation to gravity. AVhen maintained in a horizontal position during their growth the side which is uppermost becomes the dorsal surface, the other the ventral surface. Again, Sachs has shown that the young shoot of Tropxolum is radial, but that if it be exposed to strong unilateral illumination it exhibits dorsiventral properties, the more strongly illuminated side becoming the dorsal surface, and further, that the plane of dorsiventrality may be altered by causing the light to fall on a fresh side of the shoot. A similar case of the induction of dorsiventrality is offered by the thalloid shoot of Marchantia. Tracing the development of the shoot from a gemma, we find the gemma to be bilaterally symmetrical in structure and in physiological properties. It falls to the earth with one surface undermost, which becomes the ventral surface, whilst the upper surface becomes the dorsal. That the dorsiven trality is in this case induced by light is proved by the fact that, if an experiment be so arranged that the light falls on the under surface of the gemma, this becomes the dorsal surface, whilst the upper surface becomes the ventral. The further question now arises, How can the nature of the pro perties of an organ be determined, if, as has been shown, it cannot always be determined by an examination of the structure ? The answer is, that it can be determined by observing the mode of growth of the organ, and especially its response to the action of ex ternal directive influences. Thus, the spontaneous heterauxesis of a radial organ is such that each side in turn is the one which is growing with the greatest rapidity, so that, seen from above, the [VEGETABLE. apex will appear to describe a somewhat circular orbit, and its path upwards will bo spiral. This kind of nutation is termed "revolving nutation," or " circumnutation " (Darwin). Similarly, in bilateral organs each of the two sides grows alternately the more rapidly, so that, seen from above, the apex appears to oscillate from side to side of the vertical, and its path will be a zigzag line. Also in dorsiventral, as in bilateral, organs each of the two sides grows alternately the more rapidly, but the period of alternation is much longer, and may occur only once during the whole period of growth. For instance, the young leaves of Ferns are rolled up upon their internal (dorsal) surfaces ; they present what is known as "circinate vernation." Tliis is due to the fact that at first the lower (ventral) surface of the leaf grows more rapidly than the upper (dorsal) ; it is only towards the end of the period of growth that the upper surface grows the more rapidly, and then the leaf expands. Special terms have been applied to these phases of growth ; when the upper surface of the organ is growing the more rapidly the growth is said to be "epinastic," when the lower " hyponastic." The spontaneous variations in the direction of growth of an organ Extern: thus afford some indication of the nature of its organization, but directs this is more clearly shown by its response to the action of external influ- directive influences. These will now be taken in order. euces. Radiant Energy. It will be convenient to consider separately the phenomena exhibited by organs of different physiological properties. Beginning, then, with radial shoot-organs, it is usually found that Helio- when light falls upon one side of such an organ the organ curves tropisn so as to direct its apex towards the source of light, in other words, in radi; that it tends to place its long axis parallel to the direction of the organs, incident rays. The effect of the unilateral illumination is to cause heterauxesis of the organ such that the side upon which the light directly falls is the one which grows the most slowly, and therefore becomes concave. Inasmuch as all curvatures induced by light are included under the term "heliotropism," organs which exhibit the kind of curvature above described may be said to be " positively heliotropic." As examples of positively heliotropic radial organs may be mentioned radial stems, the multicellular stipes of some Fungi (Co2>rinus, Claviccps), the sporangiferous hyphse of unicellular Fungi (Mucor, Pilobolus), radial leaves, such as those of the Onion, and, as exceptional cases, some roots (Onion, Ranunculus aquatilis). In other cases the effect of unilateral illumination is the reverse of the above : the organ curves so as to direct its apex away from the source of light, though it still tends to place its long axis parallel to the direction of the incident rays. Organs curving in this manner are said to be " negatively heliotropic." This condi tion has been frequently observed in roots, and among shoots it is characteristic of the hypocotyl of the Mistletoe. Many cases of negative heliotropism in shoots have been mentioned, particularly the tendrils of Vitis and Ampelojtsis, in which its existence was first detected by Knight ; but the apparent negative heliotropism in these cases is probably the expression of something altogether different, as will be subsequently pointed out. There appear to be some well-authenticated cases of a reversal of heliotropic properties in the course of development of certain organs. Hofmeister states that the floral peduncles of Linaria Cymbalaria are positively heliotropic, but that when the fruit has replaced the flower the peduncle is negatively heliotropic ; and "Wiesner states that the peduncle of Hclianthcmum vulgare is negatively heliotropic after fertilization has taken place. The nature of the heliotropic pro perties appears thus to vary with the biological conditions of the organs. When organs are exposed throughout the whole period of their growth to unilateral illumination they usually take up a certain position which is termed the "fixed-light position," such that they curve towards the direction of incidence of the brightest light. In some cases this is not so. This may be explained by an absence of heliotropic sensitiveness, but in some cases it is due to the fact that the organs follow the daily course of the sun. Wiesner mentions Sonchi/s arvcnsis as a striking example of the latter condition. The activity of the curvature stands in a direct relation to the intensity of the incident light. The same botanist has found that for the organs of each plant there is an optimum intensity of light which produces the maximum of heliotropic effect, and that any increase or diminution of this intensity is followed by a diminished heliotropic effect. With regard to the relative heliotropic effect of rays of different wave-length, it has long been known that the rays of high refrangi- bility are much more powerful than those of low refrangibility. Wiesner finds the distribution of heliotropic effect in the spectrum to be more exactly this : the greatest curvature is produced by the rays at the junction of the ultra-violet and violet ; from this point the heliotropic effect diminishes until, in the yellow, it disappears ; it begins to manifest itself again in the orange, and increases until it reaches a small secondary maximum in the ultra-red. Helio tropic effect is by no means confined to the luminous rays of the spectrum. Wiesner, and more recently Wortmann and Barthe-
lemy, have shown that the dark heat-rays possess it. The curva-