THE LAYING OF WHEAT. 289 



of the haulms and leaves. The somewhat thoughtless assumption that the rigidity 

 of the haulms of cereals is essentially promoted by the silica which they contain, 

 impelled agriculturists, thirty years ago, to manure their wheat fields with costly 

 preparations of silica, hoping thereby to prevent the laying of the wheat — by 

 laying is understood the giving way of the lower joints of the haulms, especially in 

 continuous rainy weather, so that whole wheat fields have their haulms laid flat on 

 the earth before the ripening of the grain. In my Experimental Physiologie (1865), 

 and still earlier in my lectures, I demonstrated the incorrectness of this view, and 

 showed that the laying of the wheat has nothing whatever to do with the silica, 

 but is due to deficient lignification of the supporting tissues in the haulms, 

 which stand too closely together, shade one another, and so assume the diseased 

 condition of etiolation, to be described later ; and just those wheat-fields best sup- 

 plied with food, and the plants of which shade one another to the greatest extent, 

 are most exposed to laying. It would not now occur to any agriculturist to dig 

 his gold into his wheat-fields in the form of siHca\ The physiological significance 

 of the silicification of cell-walls is best illustrated by the observation that in numerous 

 plants the deposition of silica begins in the hairs, and especially in the prickles, 

 and extends centrifugally from the base of these to the surrounding epidermis cells. 

 This is particularly well seen in the Hop, Hemp, Gourd, etc.; and takes place 

 on stems and leaves already fully grown, or nearly so. 



We now know, therefore, that the silica so common in plants, although it 

 may occasionally be of some advantage, is nevertheless not a nutritive material in 

 the narrower sense of the word — i.e. it takes no part in the chemical processes 

 of assimilation and metabolism. We know further, that the small traces of iron 

 salts are of the greatest importance in the process of nutrition of ordinary green 

 plants, since without iron the proper instrument of nutrition, chlorophyll, is not 

 formed. With this, however, our definite knowledge of the physiological significance 

 of the nutritive substances mentioned is practically exhausted : we know, to put 

 it shortly, nothing certain as to the parts played by potassium, calcium, magnesium, 

 and phosphorus in assimilation and metabolism. With respect to sulphur, we 

 know at least that it forms an indispensable constituent in the chemical composition 

 of Proteids, and is therefore necessary for the building up of protoplasm. With 

 respect to the others, however, only so much is established, that they are absolutely 

 indispensable for assimilation — i.e. for the production of organic plant-substance; 

 for researches on vegetation show that when even only one of these elements 

 (potassium, calcium, magnesium, and phosphorus) is excluded from the nutritive 

 mixture, assimilation soon ceases, and the production of organic plant-substance 

 proceeds no further. The salts of these elements, therefore, certainly take part in 

 the chemical processes which occur during the formation of organic plant-substance 

 from inorganic material, though we do not know what part they play in it. 



Moreover, it is also not to be forgotten that the elements mentioned, or 

 their compounds, take part not only in assimilation itself, but also in the meta- 



' With respect to silica in plants, and the fact that the so-called laying of wheat is caused by 

 shading and etiolation, and not by want of silica, as hitherto supposed, what is necessary is stated 

 in my ' Experimental-physiologie^ 1865, P- 150- 

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