242 PLANT AND ORGANIC CHEMISTRY 



Iron is found to be essential to green plants only. If a seed- 

 ling be cultivated by water culture in a fluid containing no iron, 

 the leaves will become pale until at length they are nearly white, 

 but on the addition of a small quantity of iron to the solution, 

 or if the white leaves are painted with a dilute iron solution, 

 they will very shortly become green. It plays an important part 

 in the formation of the green coloring- matter, though it does 

 not enter into its chemical composition. 



" Buckwheat, 1 barley, and oats do not flourish when grown 

 in solutions containing no chlorides, and as in these plants the 

 chlorophyll corpuscles become overfilled with starch grains, it 

 was thought that this element was of importance in connection 

 with the translocation of carbohydrates." 



Sodium 2 has been used in water culture to replace potassium, 

 but the plants deprived of potash did not develop. 



Manganese is abundant in the ash of Trapa natans. I also 

 found it in the different portions of Yucca angustijolia. s 



Iodine and bromine are found in marine Alga and in minute 

 quantity in some plants grown far from the sea. 



Silica is found in the form of soluble or insoluble silicic acid. 

 It occurs principally in the cell wall, but it has been found in the 

 cell sap of a plant (Equisetum hiemale 4 ), and certain cells in 

 the pseudo-bulbs of epiphytic orchids 5 contain each a plate of 

 silica. 



Experiments have shown that the absorption of silicic 6 acid 

 greatly assists the assimilation of other plant foods, and that 

 plants to which it is supplied show a decidedly more healthy 

 development of grain and straw than others not so treated. 

 Silica is doubtless of mechanical use, giving firmness and rigid- 

 ity to plant tissues; though the real cause of "laying" of crops 



1 Vines, Cambridge edition, p. 136. Also, Beyer, Landw. Versuchs-Stat., xi. 

 Leydhecker, ibid., viii. 



2 Salm-Horstmar, Knop and Schreber. 



8 "Yucca Angustifolia," Helen C. De S. Abbott, Trans. Am. Philos. Soc., 

 Dec. 18, 1885, also ante, p. 126. 



4 Lange, Ber. d. Deutsch. Chem. Ges., xi. 



6 Pfitzer, Flora, 1877. 



8 C. Kreuzhage and E. Wolff, Landw. Versuchs-Stat., xxx, 161-198 (Jour. 

 Chem. Soc., 1884, p. 1112). 



