114 THE FUNCTIONS OF 



water lilies and other species in presence and absence of boron 

 are of interest. He found that when the pollen was placed in a 

 drop of nectar from the flower, germination was normal, but 

 when placed in a drop of sugar solution of the same concentra- 

 tion, the grains either failed to germinate or the pollen tubes 

 produced quickly burst. But the pollen tube remained intact 

 and its growth continued if the sugar solution contained 0-001 

 or 001 per cent of boric acid. From such observations Schmucker 

 not only concluded that boron influenced the absorption of 

 water by the protoplasm but that along with sugar it played some 

 part in the formation of pectin in the cell wall. With regard to 

 the latter supposition we have already noted that one of the 

 most general features of boron deficiency is the breakdown of 

 parenchymatous and other thin cell walls, especially those of 

 the apical meristems in which pectic substances are, by some, 

 supposed to predominate owing to the relative importance of 

 the middle lamella in such cell walls. Other features of boron 

 deficiency, such as discoloration of cell walls and the brittleness 

 of petioles and leaf laminae, might also be held to support this 

 view. On the other hand, Dennis (1937) has pointed out that 

 discoloration of the cell wall is not confined to cases of boron 

 deficiency, and he suggests that the effect of boron deficiency 

 on the cell wall may be part of a more far-reaching effect of this 

 deficiency on carbohydrate metabolism. 



The view that boron as boric acid increases the intake of 

 cations and decreases that of anions was put forward by Rehm 

 (1937) as a result of experiments with Impatiens balsamina 

 grown in water culture. The cultures were provided either with 

 single salts or complete nutrient solutions with and without 

 boron. The intake of the different ions was determined by 

 analysis of the solution with the results shown in Tables XI 

 and XII. 



These data certainly indicate an increase of cation absorption 

 and a relative or absolute decrease of anion absorption as a 

 result of the presence of boron. At present it would seem to be 

 extremely doubtful whether such an effect of boron can be 

 accepted as a general one. 



There is, however, much more evidence of a relation between 

 boron and calcium. In a series of water-culture experiments 



