96 James Small on 



with acid was also done with .sunflower set^dlins-s, and marked 

 downward curvatures were obtained with these also\ 



The Breathing of Plants as the Cause of their 

 Erectness. 



In the first lecture on the erectness of plants we considered 

 how the root grows downwards because it is relati\elj' acid, while 

 the stem grows upwards because it is relatively alkaline. If this 

 difference in the reaction of the two main parts of the plant is a 

 fact, we should be able to find some general explanation of such 

 a fundamental phenomenon. 



The most generally occurring acidic substance in plants, 

 which might vary in quantity, is carbon dioxide. All protoplasm 

 in breathing gives off that gas, so that we have in respiration a 

 process which yields sufficient carbon dioxide to make the cyto- 

 plasm relatively acidic. On the other hand the green parts of 

 the plant, leaves and stems, in the presence of light use carbon 

 dioxide in the building up of sugars and starches by the process 

 called photosynthesis. This, of course, does not take place in 

 roots. Stems, however, turn upwards in darkness, as well as in 

 the presence of light, so there must be some other relevant 

 difference between stem and root. If we examine the leaf or 

 stem we find numerous large air-spaces amongst the cells, and 

 these spaces communicate with the outer air through numerous 

 small pores (stomata). If we examine the tip of the root, we 

 find very much smaller air spaces amongst the cells, and these 

 spaces do not communicate with the outer air directly but are 

 connected with the comparatively distant air-spaces of the stem. 

 Although the pores in the skin of the stem are small, they have 

 been shown to be so efficient as gas-passages that 1-4 per cent, of 

 the total surface in the form of numerous small openings lets 

 through as much gas as would pass through half of the total 

 surface in the form of one pore. 



' I'hoLograiihic lantern slidea of these results were shown. 



