Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/71

Rh VEGETABLE.] PHYSIOLOGY 61 estimates the force of downward growth of the radicle at lb, and its lateral pressure, in particular cases, at 8 and 3 Ib respectively. The evidence leads to the conclusion that the Darwinian curvature of roots is not the expression of sensitiveness to contact, but that it is the result of injury of one side of the root. Combined Effects. Now that the influences which determine the direction of growth have been individually considered, it is possible to account for the characteristic positions taken up by organs in the course of their development, in dealing with this subject it is convenient, as Sachs suggests, to classify organs, according to their ultimate position, into two groups, those which, under normal conditions, have their long axes vertical and those which have their long axes more or less inclined to the vertical ; the former Sachs terms &quot; orthotropic&quot; organs, the latter &quot;plagiotropic.&quot; The direction of growth of plant-organs under normal conditions is the expression of the resultant elFect of various external directive influences. To illustrate this in the case of orthotropic organs, let us consider the primary shoot and the primary root of a seedling growing under conditions which may be taken as normal. In the case of a shoot growing upwards into the air when light falls verti- callv upon it, its vertical upward growth is chiefly due to the action of gravity, that is, it is the expression of the particular degree and quality of the geotropic sensitiveness of the shoot. Since the light is equally intense on all sides of the shoot, it exerts no direct ive influence. Orthotropism is then mainly due to negative geo- tropism. That this is so can be readily proved in various ways. For instance, the hypocotyl of the Mistletoe, as mentioned above, is not geotropic at all ; hence it cannot be included among either orthotropic or plagiotropic organs, for it may grow vertically or it may grow obliquely, its direction of growth being determined chiefly by its somatotropism. Again, when a normally orthotropic organ is grown in darkness on a clinostat its direction of growth is hori zontal. Passing now to the case of primary roots growing in the earth, when the conditions are normal that is, especially when the earth is uniformly moist around the root their direction of growth is vertically downwards. This is chiefly due to their strong positive geotropism. Let us suppose, now, that the conditions of growth of these organs are somewhat different from those which we have regarded as normal ; let us suppose that the shoot or the root is exposed to lateral light, or that the soil about the root is not equally moist on all sides. In the former case, the action of light will tend to induce heliotropic curvature, but it will depend upon the relative strength of heliotropic and of geotropic sensitiveness whether or not a curvature actually takes place. In the case of most ortho- tropic shoots a curvature (positive) would take place, thus showing the heliotropic sensitiveness of shoots to be greater than the geo tropic, but in some instances it would not take place ; in the case of most orthotropic roots no curvature would take place, but in some instances (Sinapis alba and others) a curvature (negative) would take place, showing that in most cases the heliotropic sensitiveness of roots is less than their geotropic sensitiveness. The unequal moisture in the soil around the root would cause hydrotropic curva ture, inasmuch as the sensitiveness of roots to the influence of moist surfaces is greater than their sensitiveness to gravity. We will deal with plagiotropic organs in a similar way. The majority of such are lateral members, as branches, leaves, &c. The direction of growth of a lateral member, certainly of branches of stems and roots and probably also of leaves, is at first determined by its relation to the parent axis. It has been found by Dutrochet, Sachs, and others that at their first development the long axes of lateral organs make a definite angle termed the &quot; proper angle &quot;- with the long axis of the parent organ. Dutrochet thought that the proper angle was in all cases a right angle, that the relation of lateral organ to parent axis was of the nature of somatotropism ; but this is a too general statement. The original direction of growth of a lateral organ determined by its proper angle would be maintained, in the absence of internal directive influences, by its rectipetality, but in nature it is affected by light, by gravity, &c. Lateral shoot -branches, for example, are either inherently dorsi- ventral or they become dorsiventral under the influence of gravity or of unilateral illumination ; they are then diaheliotropic, though the manifestation of their diaheliotropism may be interfered with by photo-epinasty ; they are usually negatively geotropic. Their direction of growth that is, the direction of their long axes when mature is the resultant effect of diaheliotropism and of negative geotropism. In the case of lateral root-branches these are plagio tropic but radial ; they grow outwards, slightly inclined downwards below the horizontal ; as they grow in the dark, assuming that the moisture of the soil around them is uniform, their direction of growth is affected to some extent by their slight positive geotropism. Though their geotropic sensitiveness is slight, their hydrotropic sensitiveness is great, so that their direction of growth is often very much modified by their coming into relation with moist areas of soil. A complicated case of the action of a number of directive in fluences is afforded by climbing stems, and it may be worth while to specially consider it. When the stem is young and extends only a few inches above the ground it appears to be growing almost vertically upwards, but as it elongates the last-formed internodes exhibit well-marked circumnutation. It continues to grow upwards mainly in virtue of its negative geotropism, the direction of its growth being little, if at all, affected by light in consequence of its low degree of heliotropic sensitiveness. If now one of the yoxmg growing apical internodes comes into contact with a vertical support it begins to twine around it in virtue of the sensitiveness to per manent though slight pressure which, as mentioned above, these organs possess, the direction of the curvature round the support being also that of circumnutation. The coils formed are nearly horizontal when the support is thick and become more nearly ver tical as the support grows thinner ; in any case, the steepness of the spire always increases after it is first formed, its diameter is thus diminished, and the stem gains a firm grip of the support. As the stem twines round the support it undergoes torsion around its own axis, so that any one side maintains throughout the same position, whether it be directed inwards, towards the support, or outwards or laterally. The direction of torsion may be either the same as that of coiling or the reverse, that is, either homodromous or antidromous. The direction of torsion appears to depend princi pally on the relation between the thickness of the climbing stem and that of the support, and on the smoothness or roughness of the surface of the support ; when the support is relatively thin the torsion is homodromous, but when it is relatively thick the torsion is antidromous ; with smooth supports, up to a certain limit of thickness, the torsion is homodromous, and with rough supports, down to a certain limit of thinness, the torsion is antidromous ; in a word, the direction of torsion is determined by the degree of friction between the climbing stem and the support. 2. Movements. We pass now to the consideration of movements Proto- other than those associated with growth, and we take first move- plasmic ments exhibited by protoplasm. These may be classified into two move- categories, (1) those which are performed by naked protoplasm, ments. by protoplasm, that is, which is not enclosed in a cell-wall ; (2) those exhibited by protoplasm enclosed in a cell-wall. The move ments of naked protoplasm are effected in two wavs, either by the protrusion of portions of the protoplasm, termed &quot;pseudopodia,&quot; or by permanent flagelliform protoplasmic filaments, termed &quot;cilia&quot;; the first kind of movement is known as &quot;amoeboid,&quot; the second as &quot;ciliary &quot; movement. The amceboid movement is exhibited, though rarely, by isolated cells for instance, by the zoospores of the Myxo- mycetcs and characteristically by those large aggregates of cells which constitute the plasmodia of this group of Fungi. The pseudo- podia are thrown out at first as protrusions of the denser hyaline outer layer of the mass of protoplasm, the ectoplasm, and into this the more watery granular internal protoplasmic substance, the endoplasm, gradually flows. The repeated formation of pseudopodia in any given direction will result in locomotion taking place in that direction. The ciliary movement is characteristic of zoospores and of antherozoids. In some cases the organism, as in the case of Volvox and Pandurina, passes a large part of its existence in the mobile condition, and then the protoplasm is enclosed within a cell-wall which is perforated by the cilia. The number of cilia may be only one ; more commonly in zoospores it is two, and sometimes four ; occasionally the cilia are numerous, as in the zoospores of Vauchcrin and CEdogonium ; in antherozoids they are usually numerous. The cilia are constantly performing a lashing movement, which causes the organism to move forward and at the same time to rotate on its own axis. In considering the movements of protoplasm when enclosed within a cell-wall, the typical structure of a plant-cell, as described at the beginning of this section (p. 44), must be borne in mind. In many cells the vacuole is found to be traversed by protoplasmic filaments which extend between one part of the primordial utricle and another. These filaments are continually varying in number, in position, and in size ; they are formed and withdrawn in the same manner as the pseudopodia of naked masses of protoplasm. This kind of movement is, in fact, amceboid movement exhibited by protoplasm enclosed within a cell- wall. In all actively living proto plasm, whether naked or enclosed in a cell-wall, a streaming of the more fluid endoplasm can be observed, the direction and rapidity of the current being clearly shown by the granules which are carried along in it. This is very conspicuous in closed cells (as in leaf-cells of VaUisncria spiralis and root-hairs of Jfi/drocharis Morsus lianas) when the whole of the endoplasm rotates in a con stant direction. Movements of Mobile Organs. With regard now to the movements Move- exhibited by mobile organs, to the &quot;movements of variation &quot; as ments of they are sometimes termed, sometimes they are spontaneous, like varia- the protoplasmic movements just considered ; in other cases they tion. are only performed in consequence of stimulation : they are induced. Instances of spontaneous movements of variation are, for reasons to be given hereafter, comparatively rare. A case in point is afforded by the Telegraph Plant, Hcdysarum (Desmodium) gyrans. Under favourable conditions, particularly of temperature, the two lateral leaflets of the trifoliolate leaf move upwards and downwards, their