THE GROWTH OF THE SHOOT FROM THE BUD 47 



of light accounts for the great variety in the arrangement of 

 leaves or stems. In temperate regions, where light is often diffuse 

 and always less intense than in tropical regions, the blade of the 

 leaf is usually spread out to the light, and one leaf must not be 

 exactly above another. The arrangement of leaves on the stem 

 may easily be observed in some of our common plants. In the 

 Dead-nettle, the leaves are two opposite each other ; the next pair 

 of leaves is not on the same sides of the stem as those immediately 

 below or above them, but on the other two sides, and in this 

 way the lower ones are not deprived of light by those above. The 

 best way of obtaining some idea of the very varied arrangement 

 of leaves is to examine several plants and to notice that those 

 leaves which may be said to lie almost exactly above each other 

 are borne by the stem at some distance from each other, and there 

 may be two, three, six, eight, or even as many as eleven leaves 

 between those which are situated immediately above each other. 

 Teachers may suggest, as a lesson in observation, the finding 

 plants in which the leaves are arranged in at least six different 

 ways. 



(i.) CARBON ASSIMILATION. Light is essential for the develop- 

 ment of chlorophyll. That it is the chlorophyll which is 

 concerned in the work of carbon assimilation may be easily 

 demonstrated, but even when chlorophyll is present, leaves will 

 not assimilate carbon except under the influence of light. The 

 fact that carbon assimilation is going on in a leaf generally 

 shows itself in the formation of starch. When starch is present 

 in a leaf, carbon must have been assimilated, for carbon is 

 one of the constituents of starch. Green leaves are always 

 making starch in sunlight, and it is easy at any moment to find 

 out whether starch is present in a leaf. Boil it in a test-tube, 

 then place it in methylated spirits to get out the green colour, in 

 order that the starch may be more easily seen. As soon as the 

 leaf looks colourless, place it in iodine, and if starch is present the 

 leaf will become dark blue. Instead of taking a green leaf, a 

 variegated leaf may be chosen and tested with iodine ; the leaf 

 of the variegated Ivy will do. The green part of the leaf becomes 

 dark blue, the rest does not. This little experiment shows that 



