516 ORGANOLOGY. 



existing in the plant is changed into sugar and gum, and that with 

 that change the development of the new leaves and buds takes 

 place. In perennial dicotyledonous trees, it is followed by the 

 formation of the new yearly rings. How the single cells assimilate 

 their own sap, is only very generally determined in each species of 

 plants : in the light, they form much mucus, chlorophyll, and bitter 

 extractive (tannic acid) ; excluded from light, more gum, starch, 

 and sugar. Definite compounds, according to the specific variety 

 of the cells, and always as simple matters (volatile oils, fixed oils, 

 gum, and jelly), are discharged into the intercellular passages, 

 and form the very different kinds of milky juice seen in the milk- 

 passages and milk- vessels. The process of this inward excretion is 

 yet unknown. 



Lastly, the following fact must be noticed : All the fluids in 

 particular cells (as in the pith and spiral vessels) are withdrawn, or 

 the cells (as parent cells), or masses of cells (as those of the ovule), 

 are through chemical processes dissolved, and this fluid is absorbed 

 into the general masses of sap. This process, which is yet entirely 

 unexplained, is called Resorption. 



In Vegetable Physiology no part of the science is so much in its 

 infancy as the study of the motion of the sap, because of the aimless 

 and unsuitable experiments and analogies with which unhappy caprice 

 has retarded the progress of the science for a century and a half. The 

 oldest unprejudiced observers, Malpighi, Grew, &c., furnished with the 

 necessary physical knowledge, observed that the spiral and porous vessels 

 only contained air, and named the former trachece. Then came, at the 

 beginning of the last century, Magnol, with the unlucky idea of putting 

 parts of plants which had been cut off into coloured fluids, and there- 

 from drawing conclusions. That parts of plants which have been cut 

 offtake up fluid in their spiral and porous vessels, was made the founda- 

 tion for all the idle theories that have been broached concerning the 

 circulation of the sap in plants, and for the false analogies proposed 

 between them and the higher animals. This resulted in the drawing up 

 of a complete account of the motion of the sap, which had no foundation 

 but in the imagination of its authors. The crude sap was conveyed by 

 the woody bundles up into the leaves, where it was assimilated, and from 

 thence was carried downwards into the bark, in order that the cambium 

 might be separated, and the elongation of the roots effected. It is 

 grievous to pass through the history and literature of the science, and to 

 see with what nonsensical absurdity men spun in their heads fancies, 

 which they endeavoured to prove to be actual facts. The greater part 

 of this error depends on an almost entire neglect of fundamental 

 microscopic investigation. But in recent times, with improved instru- 

 ments and methods of investigation, prejudice is decreasing, and our 

 efforts are encouraged and not overpowered. The most remarkable 

 example of the kind is Treviranus : in his chapter on the vessels he 

 says, most justly, "I have never, on examining the vessels immediately 

 after their separation from the woody bundles, perceived in them any 

 thing but air." He next gives the accurate observations of others, 

 the striking testimony of Bernhardi and Bischoff, to the same facts : 

 he appeals to the evidence of those whose only wish is to investigate 

 cautiously. When speaking of the motion of the sap he, however, 



