GEOTROPISM I. 



431 



The movements of orientation of the plant we term ' tropic ' curvatures, 

 and the capacity it possesses for producing such curvatures we term ' tropism ' 

 (p. 428). According to the nature of the external cause we may distinguish 

 tropic movements due to gravity, hght, &c., or, in other words, we may speak 

 of geotropism, phototropism, &c. In the present lecture we have to deal with 

 geotropism, and from what has been said it will be apparent that we may recognize 

 two varieties of geotropism : positive geotropism — such as are exhibited by roots 

 and all other organs whose direction of growth is towards the earth's centre, and 

 negative geotropism as manifested by shoots and such other parts of the plant 

 as grow away from the earth's centre, parallel to a radius from it. Although 

 root and shoot may be considered as characteristic organs illustrating the 

 two types of geotropism, it would be quite incorrect to assume that the geo- 

 tropic reaction was determined by the morphological nature of the organ. 

 The nature of the reaction is rather determined by the necessities of the plant, 

 and hence we meet with roots which are negatively geotropic, and grow out 

 of instead of into the soil (e. g. the pneumatophores of palms, &c., Karsten, 

 1890) and positively geotropic shoots which burrow into the soil or at least grow 

 in a downward direction (e. g., rhizomes of Yucca and Cordyline, and many 

 flower stalks after pollination, &c.). Nor is the type of geotropism always 

 constant for the same organ, for, as we shall see later, a normally positively 

 geotropic organ may become negatively geotropic and assume some other 

 relationships to the direction of the action of gravity, to which we have not as 

 yet made any reference. On the whole it may be said that geotropism is 

 a phenomenon of wide distribution in the plant world, for we meet with it not 

 only in the highest plants but also in mosses, Algae and Fungi ; it appears 

 both in multicellular structures and in unicellular organs (internodes of Nitella, 

 rhizoids of Chara), and in unicellular (coenocytic) plants such as Mucor and 

 Phycomyces. On the other hand, some plants, such as the mistletoe and many 

 Algae, are not geotropic at all. 



Our next task must be to examine more closely the precedent phenomena 

 of geotropic curvature. That this movement depends on the unequal elonga- 

 tion of opposing sides of an organ is self-evident, but how this arises cannot 

 be deduced from the actual curvature itself. Curvature, as we have seen in 

 the preceding lectures, may arise from turgor or from growth. Geotropic curva- 

 tures, as a matter of fact, arise in both ways, but curvature due to changes in 

 turgor occurs only in organs which we do not propose to discuss in this lecture. 

 We will confine ourselves at present to a consideration of those which are due to 

 growth, and endeavour to explain the principles of the subject by reference to 

 a few examples. We will select for that purpose multicellular organs, for there 

 are no exact researches available in unicellular organs. Geotropic curvatures 

 have served far more frequently as a means of demonstrating theories than as 

 the subject-matter of exact observation. Almost all that has been done in this 

 latter relation is due to Sachs. There is no reason to suppose, however, that 

 there are any differences in this respect between unicellular and multicellular 

 organisms. 



Let us begin with geotropic curvature in the tap root, which we will imagine, 

 to begin with, is laid horizontally. Fig. 133 (Sachs, 1873) shows the different 

 stages in geotropic curvature taken up by the root of Vicia faba grown in very 

 loose soil at a temperature of 20° C. The growing region (p. 289) is divided into 

 five equal parts, each 2 mm. long, from the growing apex backwards ; these 

 may be indicated by the numerals I (from to i), II, III, IV, V respectively. 

 A pointed paper index points to o (^). In S the same root is figured an hour 

 later ; the root is still straight, but it has already elongated about i-6 mm., as 

 the change in the position of o shows. In C the root, after two hours' interval, 

 is seen to have developed still further and to have curved considerably. If the 



