150 SOUTH AFRICAN BOTANY 
growth. Heliotropism is, then, the response made by 
an organ, as regards the direction of its growth to the 
influence of light. The parts of a plant are said to be 
PosItIveLy or NEGATIVELY HELIoTROPIC according 
as they turn towards or away from the light. As a 
rule, stems grow towards the light, roots away from it, 
and leaves tend to place themselves at right angles 
to the incident rays. This is known as D1a-HELto- 
TROPISM. 
127. Experiment 23.—Grow a Geranium or Sunflower 
plant in a window, so that it gets light from one side 
only. In a short time, it will be found that the stems 
have all turned towards the light, and the leaves at 
right angles to it. 
128, Experiment 24.—Pin a Bean seedling to a cork in 
a large jar with a little water in it. Cover this jar with 
black paper, leaving a slit down one side. In a few 
days the shoot will be growing towards the slit, and the 
root away from it. 
129. Gravity.—The response made by plants as re- 
gards their direction of growth to the stimulating in- 
fluence of gravity is known as GEoTrRopIsM. As before, 
we speak of positive and negative geotropism. Primary 
roots are PosITIVELY GfEOTROPIC, 1.e. they grow towards 
the centre of the earth; Secondary roots are Dta- 
GEOTROPIC, and stems are NEGATIVELY (JEOTROPIC. 
130. Experiment 25.— Line a test-tube with blotting- 
paper, fix a Pea or Mealie seedling between the blotting- 
paper and the glass, and then wet the blotting-paper, 
and keep it damp. As soon as the radicle and plumule 
have come out, turn the test-tube upside down. It will 
