184 The Roots of the Wheat Plant. 



others far too numerous to be entered upon in this place. New 

 varieties of wheat are constantly becoming the fashion with the 

 agriculturist, but it must not be concluded that this is the result 

 of caprice, as it is the nature of derivative plants to lose some of 

 their qualities after a long career of changes, and hence varieties 

 are always useful as a change ; and the more distinctive these are, 

 if adapted to our soil and climate, the better. 



3. Development : to what extent affected hy top dressings at 

 various periods of growth. — It would be useless to reproduce in 

 this place the many analyses that have from time to time been 

 made of wheat, both as respects its straw and grain, or to attempt 

 to show from these that the wheat-plant must obtain the in- 

 gredients of its structure from the media by which it is surrounded, 

 as these are points which are settled both by practical experience 

 and theoretical reasoning. We now know that where chemical 

 matters are constantly being taken from the soil in crops, so must 

 they be as constantly returned either by direct or indirect means, 

 and in the growth of wheat we can apportion such substances 

 either with a view to straw or grain in particular, and indeed 

 almost determine for the production of an excess of a particular 

 substance in either. 



With respect to crops in general, direct manurinr/ to encourage 

 their growth is usually adopted. For the cereals, however, and 

 especially wheat, this plan is seldom deemed advisable ; but pre- 

 vious crops or a dunged winter fallow Avith roots are usually but 

 preparatives in practice for the wheat crop, while special manures 

 ai'e used in various stages of growth or exigences in the progress 

 of w*heat. 



Again, without attempting to decide the question in dispute as 

 regards the mineral theory of the Baron Liebig, or the nitrogenous 

 vianures as experimented and written upon by JNIessrs. Lawes 

 and Gilbert, we yet cannot avoid the conclusion that special 

 nitrogenous manures applied to the growing wheat are for the 

 most part remunerative, while the pure mineral manures are not : 

 indeed the dictum, laid down of mineral manures being best 

 adapted for roots and nitrogenous kinds for cereals, may, I think, 

 be deemed a settled matter in practice. Admitting, then, this 

 conclusion in its simplicity, I would here beg to point out a 

 reason for the employment of special nitrogenous manures in 

 cereal crops founded on an examination of the structure of the 

 wheat leaves on the one hand, and of those of the turnip on the 

 other, which, though it may in all probability affect future dis- 

 cussions upon this subject, arc yet not meant as arguments in op- 

 position to, or support of, one theory or the other. 



If we examine the epidermis of the leaf of wheat both from the 

 upper and under side with a magnifying power of about 250 



