188 



BOTANY 



almost every ash. As might be inferred from the irregular occurrence 

 of many of the elements, they are not all necessary for nutrition, and 

 although their occasional presence in a plant may sometimes change 



certain of its special character- 

 istics (thus the presence of zinc 

 produces the so-called calami ne 

 varieties, for example, Thlaspi 

 alpestre var. calaminare, Viola 

 lutea var. calaminaria (Fig. 182), 

 etc.), they do not exerciseadeci- 

 sive influence upon its existence. 

 The Essential Constituents 

 of Plant Food. Chemical 

 analysis, while enabling us to 

 determine the substances pres- 

 ent in plants, does not show 

 how far they are essential for 

 nutrition. From culture experi- 

 ments, in which the plants are 

 grown in a medium of which 

 the constituents are known, and 

 kept under chemical control, it 

 nas ^ een ascertained that, in 

 addition to carbon, hydrogen, 

 oxygen, and nitrogen, which 

 form the principal part of the 

 combustible elements of the 

 dry substance of plants, sul- 

 phur, phosphorus, potassium, 

 calcium, magnesium, and iron 

 are absolutely indispensable to 

 the growth of all green plants. 

 In the absence of even a single 

 one of these elements no normal 

 development is possible. 



According to MOLISCII, only nine 

 FIG. 182. Viola lutea var. calamity, ;<>. which has of these elements are required by the 

 longer stems, smaller flowers, and is more branched ]? lln gi It i s not however iron 

 than the type form. (i nat. size, from SCH.MPKK'S gh be ' supposed, but ' either 



Plant-GeograpJiy.) 



calcium or magnesium, that is 



unessential. On the other hand, 



the ten substances named suffice for the nutrition of most green plants ; but 

 it is not to be denied that certain other substances are of use in the plant 

 economy and of advantage to growth, although not indispensable. Thus, for 

 example, Buckwheat flourishes better when supplied with a chloride, and the 

 presence of silica is advantageous as contributing to the rigidity of the tissues. 



