158 LECTURE IX. 



more convenient to consider it when we are in a position to 

 discuss the whole question of the relation of the metabolism 

 of plants to external conditions (Lect. XIII.). 



We may consider, in conclusion, the formation of organic 

 substance by the plant in its more general aspect. We have 

 found that the constructive metabolism of the green plant, 

 like that of all living organisms, has, as its end, the formation 

 of the extremely complex substance which we have termed 

 protoplasm ; this is destined to undergo decomposition into 

 simpler substances, some of which are and some are not of 

 use to the plant, the decomposition being accompanied by 

 the setting-free of energy in the plant. We have seen that the 

 materials which a green plant has at its disposal for this pur- 

 pose are of very simple composition, carbon dioxide, water, and 

 salts containing nitrogen and sulphur. The steps of the pro- 

 cess we have traced as closely as our information will allow; 

 the first is probably the formation of comparatively simple 

 substances containing C, H, and O (perhaps formic alde- 

 hyde and its polymers) ; then the formation of more complex 

 substances containing N in addition (asparagin, leucin, etc.) ; 

 and finally, by further synthesis, of still more complex 

 substances (proteids). We have found that the starch which 

 makes its appearance in the chlorophyll-corpuscles when 

 constructive metabolism is in active operation, is not the first 

 product of the synthetic processes, but only an indirect 

 product : protoplasm is the substance which is formed in the 

 chlorophyll-corpuscles, and it is only in consequence of the 

 decomposition of the protoplasm formed that starch is pro- 

 duced. We will consider the nature of this decomposition in 

 detail in a subsequent lecture. 



We may enquire, too, if the constructive metabolism 

 is equally energetic in all green plants. We should be 

 inclined to say a priori that this is not the case, a decision 

 which is confirmed by Weber's comparative observations on 

 the leaves of certain plants. He determined the amount of 

 organic substance formed in 10 hours by one square metre of 



