414 Vegetable PJiysiology. 



portionate to the formation of chlorophyll. But we are arrested 

 here. A multitude of questions at once crowd upon us, to 

 which we cannot give any answer. Does the assimilation take 

 place in the formation of the waxy matter? What is the import 

 of the green colouring principle ? Is all the assimilated matter 

 first combined in the chlorophyll-granules, and then decomposed 

 in the dark into dextrine, &c. ? Or do the granules work by 

 " contact-action," whatever that may really signify ? Further, 

 does the soluble assimilated matter, which may exist as dextrine 

 and sugar, when formed by direct assimilation, or by decomposi- 

 tion from starch, fixed oils, or soluble cellulose (amyloid), merely 

 mix with, or enter into combination Av^ith the albuminous sub- 

 stances, which use it for reconstruction in other situations? That 

 it enters into mixture at least is evident by the way in which 

 cellulose, starch, &c,, are formed upon its free surfaces by a kind 

 of excretion ; there is no contact-action upon a surrounding medium 

 when starch-granules are formed in cavities of the substance of the 

 albuminous matters, any more than when watery-cell-sap, con- 

 taining dextrine or sugar, is exuded into newly formed vacuoles: 

 still less when a cellulose membrane appears suddenly upon the 

 surface of the free mass of protoplasm, lying in water, as in the ger- 

 mination of the zoospores of the lower A Igae. Through what means 

 and with what changes of their own constitution the nitrogenous 

 matters are enabled to take up and lay down, as we may express 

 it, the so- varied secondary products of assimilation, are questions 

 of extreme complexity, altogether obscure at present ; and they 

 present remarkable difficulties in the way of investigation. So 

 much knowledge, however, has been attained in recent times in 

 the domain of organic chemistry, that every encouragement is 

 offered to the prosecution of these inquiries. In the mean time 

 there is abundant work for the microscope in investigating the 

 special cases of cultivated plants in their varied conditions and 

 stages of growth. This alone can furnish a key for the explana- 

 tion of the different composition of structures in different stages, 

 revealed by chemical analyses, since it is evident tliat the diversity 

 depends upon the operations which have their seat at a given 

 epoch in the cells of a given tissue, not in accidental differences 

 of a general " sap " passing through the organs. 



Through the joint efforts of the chemist and the anatomist, with 

 a more extended use of the experimental method, we may hope 

 to arrive in time at the true theory of vegetable nutrition, and to 

 solve also the problem of the " circulation " or " diffusion " oi 

 sap in plants. 



