94j anatomy and physiology of 



produce, merely products wliich have a definite purpose m tlie 

 nutrition and growth of the plant are found, or other compounds 

 arise at the same time, which are of no further importance in 

 the functions of the living plant, and must he removed from the 

 cells carrying on the vital functions of the plant. This question 

 cannot be answered with certainty so long, on the one hand, as 

 the nutritive process is so imperfectly known, that in regard 

 to the chemical processes connected with it, we possess merely 

 more or less hazardous hypotheses, but not any knowledge what- 

 ever explanatory of the details; and so long, on the other hand, 

 as we are unacquainted from physiological causes with the import 

 of a great number of chemical compounds, which occm* more or less, 

 but yet not universally diffused throughout the Yegetable King- 

 dom ; 6. g,, of the essential oils, resin, the milk-saps, the vegetable 

 alkaloids, &c., which substances are usually denominated secretions. 

 A large portion of these substances, in particular the essential oils, 

 the alkaloids, the majority of the milky juices, are in the highest 

 degree poisonous both to the plants which prepare them, and to 

 others when they are caused to absorb them. These secretions are 

 commonly separated from the other matters within the plant, being 

 either, as is frequently the case with the essential oils, enclosed in 

 special cells, or contained in canals which run between the cells, 

 as is often the case with essential oils and resin, and universally 

 with the milky juices. In the majority of plants containing milky 

 juices, these canals are lined with a special membrane, and are 

 then called milk-vessels, but can scarcely be separated from mere 

 canals destitute of proper membranes, running between the cells, 

 since true milk-sap is found in the latter in many plants, as in 

 Rhus. 



Ohserv. Althougli the theory of the milk-sap is but distantly related 

 to the subject of the present treatise, the cell, yet I cannot avoid- touch- 

 ing here upon the views propounded by Schultz, siace if they were con- 

 firmed, they would effect a complete metamorphosis of the theory of the 

 nutrition of plants. Schultz has striven, for a long series of years, in many- 

 essays (especially in ^' Die Natwr der lebenden Pflcmze^' 1833-28 ; ^'^ Sur la 

 Gircutation et sur Us vaisseaux latici/eres dans les Flantes^^ 1839 / '^ Die 

 Gy close des Lehemsaftes,'' 1841), to demonstrate a complete analogy be- 

 tween the milk-sap and the blood of animals. According to him, the milk- 

 sap is organized, and consists of a plasma becomiug coagulated out of the 

 plant, and of globules which correspond to the lymph and blood corpuscles. 

 On the coagulation of the milk-sap, an elastic coagulum, like the fibriae of 

 the blood, is said to separate, which is composed of caoutchouc, pure or 

 mingled with wax and gum, enclosing the globules of fatty or waxy mat- 

 ter, the larger of which are clothed with a membrane. In addition 

 to caoutchouc, the plasma contains sugar, albumen, gum, and salts, in 

 solution. 



In all this account of the analogous organization of the milk-sap and 

 the blood, there is not a word of truth. The caoutchouc, as I have de- 

 monstrated by the simplest experiments (" On the milk-sap and its motion.'^ 



