388 



VEGETABLE GROWTH. 



materials for its growth ; but if an adequate amount of assimilated 

 substance has been stored up, growth can go on in the dark until 

 this store is exhausted. It is, in fact, in the dark that nearty 

 all vegetable growth takes place. It is well known that all the 

 points of growth in the ordinary higher plants are more or less 

 protected from the action of light. Thus, the growing tissues of 

 buds are concealed beneath external structures ; so also is the 

 cambium by which dicotyledons increase in thickness. 



1016. When, however, a shoot develops in darkness it is apt 

 to become much more attenuated than when it develops in light ; 

 its leaves are etiolated, and of abnormal shape and diminished 

 size. Such shoots are said to be " drawn." 



1017. There is considerable difference in the degree to which 

 different parts of plants are affected by the withdrawal of light, 

 and there are also differences in this respect between different 

 species. The effect of darkness upon shoots is well shown b}^ 



the simple experiment 

 of conducting a branch 

 of some strong plant 

 like Tropseolum or a 

 gourd into a dark box, 

 all its other leaves be- 

 ing kept in the light. 

 The effects are more 

 striking when the shoot 

 is a flowering one ; 

 the inter nodes will be- 

 come much drawn, the 

 leaves will be small 

 and blanched, the calyx 

 will be pale, but the 

 rest of the flower will 

 be hardly affected 

 either in shape or size. 

 It sometimes happens, 

 however, that the flow- 

 ers will be abnormal. 



1018. The relations of growth to oxygen. All growth is accom- 

 panied by the oxidation of assimilated substance, or food. Can 

 growth be stimulated by furnishing to the plant a larger amount 

 of oxvgen than it would obtain under natural conditions ? This 



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FIG. 172. Growth of gourd in light and darkness. (Sachs.) 



