134 ELEMENTARY BOTANY 
Growth cannot take place without food; this has already 
been shown in the water-culture experiments (Chapter XVIII.). 
Seeds, bulbs, tubers, are able to grow at the expense of the 
reserve material stored up in them, but when this is exhausted 
plants must have food material supplied to them, and must be 
in contact with the air; so that they can get not only the 
oxygen, but also the carbon dioxide, which they require for 
the formation of starch. | 
Plants which live either on decaying material, or on living 
organisms, obtain their food from the substance or organism 
on which they are living. 
To sum up, the conditions of growth are: (1) The plant 
must be living; (2) the plant must have water ; (3) the plant 
must have free oxygen; (4) the plant must have food, both 
from the air and soil. | 
The rate of growth should be measured day by day. This | 
may be done in the case of the root by marking, with a 
camel’s-hair brush and Indian ink, the tip in millimetres 
(Fig. 12), and measuring exactly the additions of every twenty- 
four hours. In the same way the growing point of a stem 
may be marked and measured. 
The next series of experiments illustrates the effect otf 
external conditions on plant-life. 
Plant seeds in moist earth, grow some in the 
dark, others in the light, and note the difference. 
They must be kept watered. At the end of a month they 
may be compared. 
Or, instead of seeds, it 1s interesting to compare two 
Hyacinth bulbs of identical species. They should be put in 
hyacinth glasses filled with water, so that the bottom of the 
bulb is just above the water, towards the end of October 
or the beginning of November; both should be left for 
about six weeks in the dark until the roots are fully formed. 
Then one may be kept ina dark cupboard, the other, if possible, 
in the same room in a window. Care must be taken that 
there is always water in the hyacinth glass ; but as the bulbs 
contain food material, it is not necessary to supply them with 
Experiment A. 
