108 PLANT PHYSIOLOGY 



Silicon. — The stems of cereals are especially rich in this ele- 

 ment, the ash of corn stems being 60% silicon. The scouring rushes 

 (Equisetum) contain even higher percentages of silicon, the ash 

 here reaching as high as 70-80% of this substance. The silicon 

 of the grasses has been thought to play an important if not es- 

 sential role in the support of the long slender stems, but corn has 

 been grown for four generations without it, and experiments car- 

 ried on with wheat have led some observers to the conclusion that 

 the lodging of the grain is not due to a lack of silicon, as was 

 formerly supposed, but to crowding and insufficient illumination. 

 The lodging is thus the result of etiolation. Anatomical studies 

 by Koch (1872) have shown that lodged stems have all the char- 

 acteristics of etiolated ones, but it is reasonable to expect that the 

 presence of silicon would help to counteract the weakness produced 

 by shading. Whatever role silicon may play in supporting the 

 stem, it certainly helps to protect the plant against the attacks of 

 various plant and animal parasites. Cell walls that are impreg- 

 nated with silicon are not so easily penetrated by fungal hyphse, 

 and grains grown in the absence of silicon are much more suscep- 

 tible to the attacks of rust than those where silicon is present. 

 Similarly, plants rich in silicon have been found capable of resist- 

 ing insect attacks where those lacking it have succumbed. Lem- 

 mermann (1925) and Nemec (1927) have established that silicates 

 favor the assimilation of phosphates, and a part of their favorable 

 effect may, therefore, be attributed to this. 



Sodium. — While this element is essential for animal nutrition 

 and is very common in plants, it is not an essential one. It may 

 partly replace potassium but, as will be discussed in the following 

 chapter, this replacement value probably concerns its balancing 

 function rather than its nutritive one. To the extent that it can 

 replace potassium, it may be considered as a substitute for it, 

 and sodium may thus help to conserve the potassium supply. 



Zinc. — Some of the less common elements such as zinc are 

 found to have very specific effects. In the seed plants, the pres- 

 ence of zinc may produce certain variations of color and form 

 known as calamin varieties, as an example of which may be cited 

 a calamin variety of the pansy (Viola tricolor). Similarly a red 

 pigment develops in Bacillus bruntzii when iron salts are present. 

 Zinc has been reported necessary for normal growth of sunflowers, 

 barley, buckwheat, and beans, but to settle this point more work 



