446 



' parallelogram of forces '. Plagiotropy of this type we will discuss later on ; at 

 present we will confine ourselves to those organs which are plagiotropic under 

 the influence of gravity only. An excellent and characteristic example is the 

 horizontally growing rhizome, such as we meet with in Heleocharis palustris. The 

 fact that this rhizome grows at a certain depth below the surface demonstrates 

 that one factor which has often a far-reaching effect on the orientation of organs, 

 i.e. light, plays no part in this case (Lecture XXXVI). Since also other directive 

 agents are excluded (Lecture XXXVII) we have only gravity left, and we must 

 assume that the horizontal position taken up by the rhizome has to do with 

 geotropism in some form or another. The correctness of this conception 

 has been demonstrated by ELFVING (1880 a). He planted a subterranean 

 shoot of Heleocharis in a vessel filled with loose soil ; the vessel had one wall 

 made of glass so that the direction of the new growth could be studied. When 

 the rhizome is planted in the normal position the new region maintains the 

 same line of growth as the old ; if the apex be bent obliquely upwards or down- 

 wards the new region becomes bent sharply 

 back into the horizontal position. If, on 

 the other hand, the axis be twisted round in 

 the process of planting so that a flank or the 

 under side now faces upwards no reaction 

 of any kind follows, the rhizome grows 

 straight on horizontally without bending or 

 twisting. From these experiments we may 

 conclude that the rhizome of Heleocharis 

 grows, not as in the case of ordinary or- 

 thotropic organs, parallel with, but at 

 fight angles to, the direction of gravity; 

 and yet there is no difference between the 

 sides of the rhizome, which is radial in its 

 structure. The rhizomes of Scirpus and 

 Sparganium (ELFVING, 1880 a) and also of 

 Adoxa and Circaea (GOEBEL, 1880) have 

 been shown to behave in the same way, 

 and in all probability the majority of shoots 

 which develop in a horizontal direction in 

 the soil have the same characters (e. g. 

 Paris, Anemone nemorosa, &c.). In many 

 cases these shoots are the principal shoots 

 of the plants concerned (e.g. Paris, Adoxa), but lateral branches also exhibit this 

 special form of geotropism which we may term plagiogeotropism or diageotropism. 

 The same phenomenon is very obvious in the case of the lateral roots of the 

 first rank, which always form an angle with the rigidly orthotropic and positively 

 geotropic main root. It is quite true that in this case the angle is not always 

 a right angle, it is more often acute, and it is by no means constant in size ; 

 that it is determined by the direction of gravity was shown by SACHS (1874) by 

 simply turning the plant round through 180. He found that in a short time 

 the new growths made about the same angle with the line of direction of gravity 

 but a totally different one with the line of the chief root, and that after they 

 had been again inverted the original direction of growth was resumed. Fig. 138 

 illustrates SACHS'S experiment, the darkened parts of the lateral roots being 

 those formed whilst the plant was in the inverted position. Again, the lateral 

 roots are strongly radial, for they may be twisted at will round about their long 

 axes without their exhibiting any reaction so long as they remain at the correct 

 angle with regard to the direction of gravity. Curvature follows at once if there 

 be any deviation upwards or downwards from the specific 'limiting angle'. 



Fig. 138. Viciafaba. Chief root with lateral 

 roots, grown in soil behind a glass plate, first in 

 the normal, then in the inverted, and finally once 

 more in the normal position. The increase 

 while in the inverted position is shown in black. 

 After SACHS 11874, p. 605'. 



