VEGETABLE.] PHYSIOLOGY 59 tures produced by the dark rays arc sometimes designated by the term thermotropism. " The effect of unilateral illumination is not immediately ex hibited, nor does it cease immediately upon the withdrawal of the organ from light. If an organ be exposed for only a short time to unilateral illumination, it may not exhibit any curvature during the period of exposure, but will curve subsequently in darkness. The exposure had sufficed to induce heliotropic curvature. This Wiesner terms "photo-mechanical induction," but it is simply due to the slow response of the organ to the directive influence of the incident rays, to a long "latent period." Turning now to the part played by the growing organ in helio tropic curvature, it is clear that the curvature will largely depend upon the heliotropic sensitiveness of the organ, and it must be borne in mind that organs vary widely in this respect. The cur vature is in all cases confined to the growing region of the organ, or, to put it more generally, to the region which is capable of growing. In connexion with this point, the further one naturally arises as to the seat of heliotropic sensitiveness. It is usually assumed that those zones which are growing most rapidly are those which are most sensitive, but Darwin found that in some cases (cotyledons of Phalaris and Avena, hypocotyls of Cabbage and Beet) illumination of the tip of the organ appeared to induce heliotropic curvature. This cannot at present be regarded as fully established. With regard to the heliotropic phenomena presented by simply bilateral organs, such as the leaves of Irises, it will suffice to say that they are positively heliotropic. The heliotropic phenomena presented by dorsiventral organs are more complicated. It has been observed that dorsiventral branches (including the tendrils of Vitis and Ainpclopsis mentioned above), when exposed to vertical light, tend to assume a horizontal position, whereas in darkness they usually grow erect. Dorsiventral leaves usually remain horizontal in darkness. A full discussion of these facts cannot be entered upon here ; a few remarks must suffice. It is argued by some, especially by De Vries, that the horizontal position of dorsiventral organs when exposed to light is due to negative heliotropism ; but there is no adequate proof that this is really the case. Sachs says on the subject, "So far as I can appre hend the facts, the negative heliotropism of the Marchantia-shoot, and that of many other shoots which behave in the same way, is the same phenomenon as the epinasty of foliage-leaves described by De Vries. " Detmer has in fact found that exposure to light, quite independently of the direction of the incident rays, induces the rapid growth of the dorsal surface of dorsiventral leaves in a word, induces photo -epinasty. Continued photo -epinasty would clearly cause the organ to curve downwards below the horizontal plane, and this is occasionally actually the case. But more fre quently the organ remains in the horizontal plane when the light falls vertically upon it, or, to put the case more generally, the organ takes up such a fixed-light position that its long axis is at right angles to the direction of the incident rays. Moreover, it has been ascertained that it is always the dorsal surface of leaves which is directed towards the brightest incident light. It is clear that, in addition to the induction of photo-epinasty, light exercises a directive influence upon the growth of these organs. The attempt has been made to explain this by ascribing to the dorsiventral organs merely the heliotropic properties which belong to radial organs, but this explanation is quite insufficient. They can only be accounted for by attributing to dorsiventral organs, as Frank has done, a peculiar heliotropic sensitiveness, which he terms "trans verse heliotropism " and Darwin "diaheliotropism," which mani fests itself in the assumption of such a position that the dorsal surface of the organ is placed at right angles to the direction of the incident rays. Gravity. The influence of gravity in determining the direction of growth of an organ manifests itself in phenomena which are designated by the term " geotropism. " We will again consider sepa rately the phenomena which are presented by organs of different physiological properties. Beginning with radial organs, we find that a great number of them normally grow either upwards or downwards ; thus, primary shoots grow upwards and primary roots grow downwards. If any attempt is made to alter the direction of growth of these organs they at once curve so as to regain their normal direction. That the direction of growth is actually the result of the action of gravity was first demonstrated by Knight. He caused seeds to germinate on a wheel revolving with sufficient rapidity to set up a considerable centrifugal force, and he found that the roots and stems of the seed lings behaved with regard to the direction of the centrifugal force precisely as they do with regard to that of gravity : the roots grew radially outwards, and the stems radially inwards. He states his conclusion thus : " I conceive myself to have proved that the radi cles of germinating seeds are made to descend, and their plumules to ascend, by some external cause, and not by any power inherent in vegetable life ; and I see little reason to doubt that gravitation is the principal, if not the only, agent employed in this case by Nature. " In conformity with the terminology used with reference to helio tropism, organs which grow towards the centre of the earth are said to be " positively geotropic " and those growing in the opposite direction " negatively geotropic." As examples of positively geo tropic radial organs may be mentioned, in addition to primary roots, the hyph;e of Moulds which penetrate into the substratum, the root-like filaments of I auchcria, Caulerpa, and other A Igs, the rhizoids of Muscincae, the rhizomes of some plants, such as Yucca and Cordylinc. As examples of negatively geotropic radial organs may be mentioned, in addition to primary shoots, the stipes of Mushrooms, the sporangiferons hyphie of Moulds, the stalks of the receptacles of Liverworts, the setee of Mustincse, the peduncles of many flowers, the climbing roots of various epiphytes. Cases o; the absence of geotropic irritability are afforded by the hypocotyl of the Mistletoe, and by the aerial roots of various epiphytes. A reverse of its geotropic properties may take place in the course of the development of an organ. Vbchting has found, for instance, that the peduncle of the Poppy is negatively geotropic whilst the flower is in the bud, but positively geotropic during flowering and fruiting. In dealing with these phenomena we have first to consider the effect of gravity acting at different angles. Sachs concludes, and his conclusion is generally accepted, that the geotropic influence of gravity is greatest when the long axis of the organ is at right angles to the vertical, and that it is zero when the long axis of the organ coincides with the vertical, whether the apex point upwards or downwards, or whether the organ be positively or negatively geo tropic, that is, if the force of gravity acting at any point of an organ be decomposed into two forces, the one acting at right angle;; to the long axis of the organ and the other along it, it is only the former which produces a geotropic effect. Sachs bases this view upon the fact that geotropic curvature is more rapidly produced when an organ is horizontal than when it is in any other position. Elfving has made observations which suggest a different view, namely, that the geotropic action of gravity upon an organ is greatest when that organ is removed as far as possible from its normal relation to the vertical. The degree of geotropic sensitiveness is different in different organs. This is shown by the different directions of growth taken by different organs when grown under conditions which prevent to a sufficient extent other directive influences from producing any effect. For instance, primary roots grow vertically downwards, but lateral roots grow more or less nearly horizontally. It has been ascertained, by means of centrifugal force, that lateral roots can be induced to behave like primary roots if only the force is sufficient : lateral roots grow radially outwards on the wheel when the centrifugal force is 4g (<7 = force of gravity). It may be inferred that their geotropic sensitiveness is one-fourth of that of primary roots. The response of an organ to the directive influence of gravity, as in the case of heliotropism, is not immediate, but is preceded by a long latent period. An organ placed horizontally will not begin to curve for some time, and if then placed vertically the curvature will proceed for some time. Geotropic curvature, like heliotropic curvature, is a phenomenon of induced heterauxesis, the result being the assumption by the organ of such a position that gravity ceases to exert any directive influence upon it. Primary shoots and roots, for instance, find the vertical to be their position of rest, whereas lateral branches take up a more or less inclined position. It appears that, generally at least, the most rapidly growing zones of organs are those in which geotropic influ ence is most active. It has been hitherto generally accepted that the seat of most active curvature was also the seat of geotropic sensitiveness. Darwin recently brought forward the view, based upon the behaviour of roots with their tips cut off, that, in the root at least, the tip is the seat of geotropic sensitiveness. This suggestion has given rise to a number of researches, the results of which are so conflicting that it is impossible at present to come to any definite conclusion on the subject. With regard to the geotropic properties of simply bilateral organs (leaves of Irises, &c.) it need only be said that they are negatively geotropic. Coming, finally, to the geotropic phenomena presented by dorsi- Geotrop- ventral organs, we find that many organs which when growing ism of exposed to light have a more or less oblique direction of growth, dorsi- grow erect in darkness. This is obviously an effect of gravity, and ventral the organs are clearly negatively geotropic (examples are afforded organs, by the runners of Polygonum avicularc, Atriplex latifolia, and others, by radical leaves, and by thalloid shoots of Marchantia). But some dorsiventral organs do not grow erect in darkness. Frank mentions the runners of Fragaria lucida, lateral branches of Conifers, and many dicotyledonous shrubs and trees as examples. He finds, further, that when these organs are placed otherwise than horizontally they curve until they come to lie in that plane, and, if they are placed in an inverse position so that their normally inferior surface (ventral^ is uppermost, they twist on their own axes until the normal relation of their surfaces with respect to the
vertical is attained ; many leaves also behave in this way. Frank