36 



PHYSIOLOGY OF NUTRITION 



was able to show that when the stomatal pores are artificially plugged exchange 

 of gases is retarded. A privet leaf (Ligustrum vulgaris), the upper surface 

 of which was coated with petrolatum, decomposed 6.26 g. of carbon dioxide, 

 but only 1.92 g. was decomposed by a similar leaf coated on the under surface. 

 [Privet leaves have stomata only below, so that coating the upper surface 

 did not close the pores.] Stahl 1 arrived at the same result. Parts of the lower 

 surfaces of leaves that had been rendered free from starch were covered with a 

 mixture of one part of beeswax and three parts of cocoa butter, and the leaves 

 were then exposed to light; after being bleached with alcohol and then treated 

 with iodine, the part that had been covered was brown, while the remainder of 

 the leaf was dark blue (Fig. 18). Blackman's 2 results point to the same 

 conclusion. The size of the stomatal openings is also important. 3 



An adequate supply of water in the leaves is essential 

 to the normal progress of photosynthesis; according to 

 Sachs and Nagamatsz 4 no starch is formed by wilting 

 leaves, a fact which Stahl believed to be due to the 

 stomatal closure that accompanies wilting. This interpre- 

 tation is supported by the observation that leaves in which 

 the stomata remain open even in the wilted condition 

 (Rumex aquaticus, Caltha palustris, Hydrangea hortensis, 

 Calla palustris) still continue to accumulate starch after 

 wilting has occurred. 



Finally, an excess of salts in the soil has a retarding 

 effect upon the rate of carbon dioxide decomposition. 

 Schimper found that watering with sodium chloride solu- 

 tion caused development to cease in most plants (non- 

 halophytes), through a checking of photosynthesis. Ac- 

 F 1 g . 18 .—Privet cording to Stahl this, also, is due to stomatal closure, 

 leaf, the unshaded P or- caused Dy excess f sa i ts . if the leaves are slightly 



tion ofwhichwas J ..... 



covered with cocoa wounded so as to facilitate entrance of carbon dioxide into 

 butter during exposure t k e t i ssue starch accumulates about the wound margins. 



to light. This portion ' . " . 



shows no starch reac- True halophytes grow, though slowly, upon soils rich in 

 tion with iodine. sa j tSj since ^^ stomata do not close at all. 



§9. Nutrition of Green Plants by Organic Compounds. — Green plants 

 can also use as food organic compounds that are supplied from without. 5 This 

 form of nutrition may go on simultaneously with the assimilation of carbon 



1 Stahl, Ernst., Einige Versuche uber Transpiration und Assimilation. Bot. Zeitg. 52 : 11 7-146. 

 1894. 



- Blackman, F. Frost, Experimental researches on vegetable assimilation and respiration. — No. I. On 

 a new method for investigating the carbonic acid exchanges of plants. Phil, trans. Roy. Soc. London 

 Bi86 7 : 485-502. 1895. Idem, same title, No. 11. On the paths of gaseous exchange between aerial 

 leaves and the atmosphere. Ibid. B: 186 7 : 503-562. 1895. See Sect. IV. 



3 Kolkunov, V., Ueber die Abhangigkeit der Assimilation von der Grosse der Spaltoffnungen bei den 

 Gramineen. [Abstract in German, pp. 381-382; text in Russian.] Jour. exp. Landw. 8: 369-382. 



1907. 



* Nagamatsz, Atsusuke, Beitrage zur Kenntnis der Chlorophyllfunktion. Arbeit. Bot. Inst. Wurzburg 

 3: 389-407. 1888. 



5 Apparently carbon monoxide cannot be assimilated; see : Krascheninnikoff, Th., La plante verte assimile- 

 t-elle l'oxyde de carbone? Rev. gen. bot. 21: 177-193. 1909- 



