10 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



The terms I propose using to express the " turning " towards the stimu- 

 lus are as follows : (1) Phototropism (to diffused light), and Heliotropism 

 (to the sun) ; (2) Skototropism (to shade or darkness) ; (3) Thermotropism 

 (to heat) ; (4) Hydrotropism (to moisture) ; (5) Helkotropism (to attraction 

 of Gravitation) ; (6) Haptotropism (to touch) ; (7) Orthotropism (becom- 

 ing erect or pendulous) ; (8) Oircumnutation (bowing around). 



I avoid the usual term Geotropism, used for the influence of Gravita- 

 tion, because a " turning earthwards " may be due to shade as well ; 

 while Apogeotropism, I would regard as a synonym for one application of 

 Orthotropism ; for organs can place themselves in a vertical position 

 from more than one cause. 



Phototeopism, Heliotropism and Skototropism, or the Effects 

 of Light and Shade. 



In flowering plants, as soon as a seed begins to germinate, the root- 

 end (radicle) and shoot-end (plumule) of the embryo first begin to grow 

 in opposite directions, under the influence of external stimuli. The first 

 question is, Why do they grow orthotropically, i.e. in a vertically 

 straight line, and why does the former bend downwards and the latter up- 

 wards, if the growing seedling be placed horizontally ? As gravitation 

 is a constant force acting vertically downwards, we might naturally infer 

 that this was the cause acting on the root, and so the " turning downwards," 

 under the influence of gravitation was called " Geotropism " and the 

 turning upwards of the shoot was named " Apogeotropism." These words 

 are descriptive only, not explanatory. 



If we turn to the simplest or unicellular organisms, such as the spherical 

 spores of Cryptogams, which have no polarity or any distinction of parts, 

 we find that the two most important stimuli are light and shade. Thus 

 the first cell-division, as in the spores of Fucus, Equisetum* and 

 Ascophyllum, f is approximately at right angles to the incident light, 

 the daughter-cell facing the less illuminated half develops into the 

 root, while the other becomes the shoot-end. Sachs describes and 

 figures the shoot and gemmule of Marchantia, and the structures are 

 reversed when the usual dorsi- ventral arrangement has been made to be 

 ventri-dorsal. t Similar effects are produced on the prothallia of ferns ; 

 so that the position of the sexual arrangements are always on the shaded 

 side. If the prothallium be grown immersed in water it curves itself so 

 as to be at right angles to the light and then produces those organs on 

 the shady side. 



Chlorophyll granules illustrate both phototropism and skototropism 

 very well ; for in diffused light they spread themselves over the super- 

 ficial cell in such simple plants as duckweed ; such would be phototropism ; 

 but if the direct light of the sun be very intense they place themselves over 

 the vertical walls to avoid it. This movement is therefore skototropism 

 or apheliotropism,- whichever term be preferred. 



In the case of germinating embryos of plants which do not grow on 

 the ground, as the mistletoe and epiphytes, the roots adhere to the bark of 



* See Origin of Plant Structures, p. 197, note, 

 t Res. Gen. de Bot., I., p. 58, fig. 5. 

 % Sachs' Phys. of Plants, p. 524-6. 



