ANISOTROPY AS INFLUENCING TBE FORM OF PLANTS. 703 



Stem of which, wilh its bud and the large foHage-leaves which it produces, bores down 

 into the earth, and since it cannot there advance, being a thick mass, it pushes 

 the old, properly lower, part of the stem upwards higher and higher above the 

 soil; similarly the great contrast between the common Ivy and its allies, the 

 majority of the Araliacese, lies in their mutual difference in anisotropy: the 

 shoot-axes of the Ivy climb, whereas the others erect their shoot-axes independently 

 without climbing. The Ivy teaches us, indeed, yet another important fact, viz. that 

 the distribution of anisotropy in one and the same plant may alter at different 

 periods of life. It is well known that the leaf-shoots of the Ivy do not always cHmb, 

 but that when the plant is old enough leaf-shoots are produced which grow out 

 independently, and then form leaves differently shaped and placed, and finally 

 flowers and fruits. Another case of this kind is very common with plants which 

 form runners or stolons, of which the Stra^vberry affords probably the best known 

 example. Its sub-aerial stolons grow horizontally, a meter or so in length, and 

 at last the terminal bud changes its character : it suddenly produces large foliage- 

 leaves in the form of a rosette, and erect flowering shoots. A very similar 

 state of affairs exists in the case of many subterranean stolons, e. g. the Umbellifer 

 uEgopodium podagraria, so common in gardens, and which is on that account regarded 

 as one of the most troublesome of weeds. To quote a case of quite another kind, 

 the great difference in habit between the pyramidal Poplars, as contrasted with the 

 Black Poplar and other species, depends essentially upon the branching of the 

 crown of the tree : in the pyramidal Poplars all the branches turn sharply upwards, 

 whereas in the other species they stand off from the primary stem at large angles ; 

 and just this difference can be produced in the formation of varieties, and there- 

 fore by very small internal changes. A large number of the most different woody 

 plants, which otherwise form broad spreading crowns, produce varieties of the form 

 of the pyramidal Poplar, among others the common Juniper {Junipenis communis), 

 which even forms a third variety, which is shrubby and has the branches spreading 

 along the ground. 



However, all these statements are merely intended to show what is properly 

 understood by anisotropy ; and it is now time to examine the facts a little more 

 closely. In the investigation of natural phenomena which appear under very 

 different aspects, but in which something common to all is felt to be the important 

 and essential feature, it is always well first to introduce method so as to refer the 

 differences to the fewest types possible. This may best be done in the present case 

 by dividing anisotropic organs into two classes, which I have distinguished as 

 Orthotropic and Plagiotropic. Orthotropic organs are those which under ordinary 

 conditions of Hfe, where the surface of the soil is horizontal and the illumination 

 equal on all sides, grow perfecdy vertical upwards or downwards, as for instance 

 the primary stems of most trees, particularly of Firs and Pines, and the leafy stem 

 arising from the plumule of very many annual plants, such as Sunflower, Tobacco, 

 Flax, &c. ; and, generally, all organs which are positively or negatively geotropic 

 or heliotropic, in the way described in the preceding lecture. Thus primary roots 

 which grow vertically downwards and shoot-axes which tend vertically upwards are 

 anisotropic, but resemble one another simply in that both place themselves vertically 

 wilh reference to the place where they grow. In this they differ fr(nn all plagiotropic 



