Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/70

Rh 60 PHYSIOLOGY [VEGETABLE. accounts for these phenomena by ascribing to such dorsiventral organs a peculiar geotropic sensitiveness, which he terms &quot;trans verse geotropism,&quot; and Darwin &quot; diageotropism. &quot; De Vries has severely criticized this assumption. He regards the curvatures of shoots and branches into the horizontal plane !is being, to some extent, the expression of the negative geotropism of the stems inter fered with by the weight of the leaves, and, to some extent, also the expression of those forms of spontaneous heterauxesis termed epinasty and hyponasty which were alluded to above. Similarly he accounts for the torsions observed by Frank by ascribing them to the unequal twisting moment of the leaves on the two sides of the shoot when in the inverse position. In view of the existence of diaheliotropism, it may be regarded as probable that diageotropism exists also. But at present the case for the latter is not sufficiently made out. More experimental evidence must be forthcoming be fore the assumption of diageotropism can be regarded as fully justified. Hydro- Moisture. It has long been known that roots when brought tropism. into the neighbourhood of a moist surface curve towards it. For instance, when seeds are sown in a box of damp sawdust, the bottom of the box being perforated with sufficiently large holes, the roots of the seedlings grow downwards into the sawdust, and ultimately project through the holes. They then no longer grow vertically downwards, but curve so as to apply themselves to the moist surface offered by the bottom of the box. To these phenomena Darwin has applied the term &quot; hydrotropism.&quot; Organs which curve in this way are said to be &quot;positively hydrotrqpic &quot; ; but there are also &quot;negatively hydrotropic &quot; organs. Wortmann has observed, for instance, that the sporangiferous hyphne of Phycomyccs curve away from a moist surface. The phenomena are precisely similar to those of heliotropism and of geotropism. The curvature is in this case also the expression of induced heterauxesis. It takes place in this case also with the greatest activity in the region of most rapid growth. Darwin came to the conclusion that the hydrotropic sensitiveness of roots at least is localized in their tips, a conclu sion which, though opposed by Detlefsen and Wiesner, is so far confirmed by Molisch s results that it may be accepted as well founded. Galvano- Electricity. Elfving found that when a root is placed vertically tropism. between two electrodes it curves towards the positive electrode, that is, against the direction of the current. In one case (Cabbage) the curvature was towards the negative electrode. Miiller (Hettlingen), in repeating Elfving s experiments, found that the curvature was in all cases such as to tend to place the long axis of the root in the plane of the current, the curvature being towards the negative pole. These phenomena are spoken of as &quot;galvanotropism.&quot; Miiller found that the curvature was induced when the current traversed only the tip of the root, thus affording apparently another instance of localization of sensitiveness in the tip. Sub- The Substratum. -Dutrochet long ago observed that the hypo- stratum, cotyl of the Mistletoe, in whatever position the seed may have been placed, assumes such a direction of growth that its long axis is perpendicular to the surface on which the seed has germinated. Sachs has shown, by means of rotation on the clinostat, that this position is assumed both by shoots and roots. It is clear that the substratum exercises a directive influence upon the organs growing either outwards from its surface or inwards into its surface, but the nature of this influence has not yet been investigated. It is certainly not to be ascribed, as Van Tieghem suggested, to the mere mass of the substratum. The effect of a cube of turf or a pot of earth would vanish entirely in comparison with the attraction of the earth, in other words, with the influence of gravity. The pheno mena are designated generally by the term &quot;somatotropism.&quot; Contact. Contact. The effect of contact upon the direction of growth of an organ must be clearly distinguished from the effects of consider able pressure. The latter are of two kinds : in the one case the pressure arrests the growth of that side of the organ which is exposed to it ; in the other it excites the organ to active growth, particularly in thickness. Examples of the former effect are so common that they need not be specified ; examples of the latter are afforded by the thickening of tendrils and of climbing stems when they have firmly grasped some support. The phenomena now to be considered are such as are induced by slight pressure. Striking instances of this are afforded by tendrils. A very slight touch suffices, in the case of the very sensitive, such as those of Passiflora grncilis and of Sicyos angulatus, to induce very per ceptible curvature, which can be detected, according to Darwin, in half a minute after the touch. Twining In order to illustrate the subject adequately a brief account will of ten- be given here of the more important phenomena connected with drils. the twining of tendrils. Darwin has shown that tendrils are not sensitive during the whole of their existence ; speaking generally, they are not sensitive when they are either very young or full grown, and are most sensitive when they are about three-fourths grown. Darwin has also found that their sensitiveness is localized. In most the lower or basal part is either not at all sensitive or is sensitive only to prolonged contact. Most tendrils have their tips slightly but permanently hooked, and the sensitiveness is localized in the concavity of the hook. In some cases (Cobun scandens, Cissus discolor) they are sensitive on all sides; in Mutisia the in ferior and lateral surfaces are sensitive. As a rule, when in its most sensitive condition the tendril is actively circumnutating, so that it travels over a large area, and there is considerable probability that it will come into contact with some body around which it caii twine. When contact takes place the tendril begins to curve round the support. As it does so new points of the sensitive surface are stimulated, and the curvature increases and extends until the whole of the tendril lying between the original point of contact and the apex is wound in a spiral coil round the support. In some casts this is all that takes place. In the great majority of cases, how ever, the coiling of the apical portion of the tendril round a support is followed by the spiral coiling of more or less of that portion of the tendril which lies between the original point of contact and the insertion of the tendril upon the stem, provided that this is mechanically possible. The spiral coiling of the basal part of the tendril involves, namely, a considerable shortening, and, if both the stem and the support are immovably fixed, this shortening cannot take place. The turns of the coil are not all in the same direction ; they are grouped into two or more spires, separated by short straighter portions, the turns of any two successive spires being in opposite directions. This is a mechanical necessity asso ciated with the spiral coiling of a filament attached at both ends. The spiral coiling of the basal part of the tendril usually begins just below the point of attachment to the support. The coiling of tendrils, like all the curvatures hitherto considered, is a pheno menon of induced heterauxesis. Hence it is that the possibility of the twining of a tendril round a support depends upon the thickness of the support and upon that of the tendril. Most tendrils, inas much as they are very thin, can twine round strings, but those which are relatively thick can only twine round a support of some thickness, for there is a mechanical limit to the excess of elonga tion of the convex over the concave side. The spiral coiling of the untouched portion of the tendril has an especial interest, as it offers a striking illustration of the transmission of a stimulus. It is true that the tendrils of many plants, if they fail to come into contact with a support, likewise coil spirally ; but this is a much slower process, and only begins at the time when the tendrils are ceasing to grow and to be sensitive. Tendrils are not, however, the only organs which are sensitive to contact. Other instances are afforded by the petioles of most leaf-climbers, by shoots, and apparently by roots. In the case of sensitive shoots Dutrochet observed that the twin ing stem of Cuscuta is sensitive like a tendril. Von Mohl suggested that all climbing stems are sensitive, but both Darwin and De Vries were unable to detect the sensitiveness. This view of Von Mohl has been recently revived by Kohl, who finds that the inter- nodes of climbing stems are sensitive to a long-continued pressure which is insufficient to produce any simply mechanical effect. Darwin found the young interuodes of Lophospermum scandens, which is not a stern-climber, as also the peduncles of Maurandia semperflorcns, to be sensitive to touch, and Kerner states that this is also the case with the peduncles of many flowers (Poppy, Anemone, Ranunculus, Tulip). With regard to roots, Darwin was led to suspect, by observing Bar- the behaviour of the radicles of seedlings in their attempts to pass winia over obstacles in the soil, that the tip of the radicle is sensitive to curva contact, and that the stimulus is transmitted from this, the sen- ture. sory organ, to the growing zones behind it, in which the necessary curvature is then effected. He made experiments by attaching small objects to one side of the tip of the radicle in various plants, by touching one side of the tip with caustic, and by cutting a thin slice off one side, and found in most cases that the radicle curved away from the touched or injured side, that the curvature is pre cisely the opposite of that performed by tendrils when touched. The peculiar curving of radicles has been termed the &quot;Darwinian curvature.&quot; Darwin s conclusions as to the sensitiveness of the radicle have given rise to considerable discussion. It is clear, in the first place, as he himself showed, that radicles are not perceptibly affected by brief contact or by friction ; the contact must be pio- longed. Those who dissent from Darwin s view, such as Wiesner, Burgerstein, and Detlefsen, urge that the curvatures induced in his experiments- are pathological. It seems probable that this objection is valid. It may be admitted at once in the case of the experiments made by means of slicing the root-tip or touching it with caustic. With regard to the effect of small objects, such as pieces of card, it appears that the curvature of the radicle is due mainly, if not entirely, to the substances used in attaching them. In some cases, for instance, they were attached by a drop of shellac. It has been shown that the mere presence of the drop of shellac is sufficient to induce the Darwinian curvature, and microscopical examination has proved that the part touched by the shellac had died away. Moreover, it is known that a radicle can grow down wards against considerable resistance : it can penetrate into mer cury ; it can perforate tinfoil without deflexion ; Darwin, in fact,