CHEMICAL AND PHYSIOLOGICAL STUDIES. 181 



term heliotropism. Organs which grow towards a source of light 

 are said to be positively heliotropic, whilst those which grow 

 away from it are negatively heliotropic. In this sense stems and 

 flowers are positive, whilst roots are negative, and most leaves 

 are transversely heliotropic, setting at right angles to the rajs of 

 light. In most instances there is a certain position with regard 

 to the direction of the incident rays of light which an organ 

 takes up in preference to others, and this is known as the 

 optimum position ; thus, to refer to the lower plants, .ffithalium, 

 a mass of naked protoplasm, if subjected to powerful illumina- 

 tion, will withdraw to a position where the light is not so 

 powerful. Many leaves assume a position which permits not 

 of the maximum amount of light falling upon their upper 

 surfaces, but of just that amount which is found to coincide 

 with the requisite intensity of assimilation in these organs. Light 

 has thus a kind of tonic effect (phototonus) upon the growth 

 and position of organs. The explanation of the movements of 

 an organ caused by light is to be found in the fact that those 

 cells of the organ which are nearest the source of illumination 

 transpire most freely, and are not so turgid as the opposite 

 parts; and since turgidity favours growth, it follows that the 

 remoter parts will grow more strongly than those nearest the 

 source of light, and, by so doing, will cause a curve in the organ 

 concave towards the light incidence. In this manner arise the 

 curvatures produced in some stems and flower stalks. It is well 

 known that roots grow much faster in the dark than in the 

 light; this can readily be shown by growing water-cultures of 

 Hijacinthus in transparent white vases and in opaque ones 

 respectively, when it will be found that the roots springing 

 from the bulb in the dark jar are, after some days' growth, much 

 longer than those from the bulb in the transparent one. In this 

 connection it may be mentioned that the violet and ultra-violet 

 rays of the spectrum have most influence upon the formative 

 action of protoplasm. 



It is an interesting fact that the chloroplasts in the palisade- 

 cells of a leaf take up positions which vary according to the 

 intensity of illumination of the upper surface of the leaf; thus, 

 in very intense illumination they become arranged along the 

 side-walls, presenting only their edges to the incident rays 

 (apostrophe), whilst in medium illumination they are situated 



