25* Phytotomy in the Eighteenth Century. [BOOK n. 



only lies between each of two adjoining cells, a point which 

 succeeding phytotomists were a long time in determining. As 

 cells are formed by the secretion of drops of sap in the funda- 

 mental substance which is at first homogeneous, so vessels, 

 according to Wolff, are produced by longitudinal extension of a 

 drop in the mucilage and formation of a canal ; consequently 

 adjoining vessels must be separated from one another by a 

 single lamina of the fundamental substance. Though Wolff 

 expressly mentions the movement of the sap within the firm 

 mucilaginous substance between the cellular cavities and the 

 vascular canals, a movement of diffusion as it might now be 

 termed, he inconsistently enough thinks it necessary to assume 

 the existence of perforations in the bounding-walls of cells and 

 vessels to serve for the movement of sap from cell to cell 

 and vessel to vessel ; yet in the single case in which he 

 succeeded in obtaining isolated cells, namely in ripe fruits, 

 he was obliged to allow that the walls were closed. 



The growth of the parts of plants, according to Wolff, is 

 effected by expansion of existing cells and vessels, and by the 

 formation of new ones between them in the same way as the 

 first vacuoles were formed in the mucilaginous substance of 

 very young organs ; that is to say, the sap which saturates the 

 solid substance between the passages and cavities of the tissue 

 separates in the form of drops, which increase in size and then 

 appear as cells and vessels introduced between the older ones. 

 The substance between the passages and cavities, at first soft 

 and extensible, becomes firmer and harder with increasing 

 age, and at the same time a hardening substance may be 

 deposited on it from the sap which is stagnant in the cell- 

 cavities and in movement in the vascular passages, and this 

 substance in many cases appears as their proper membrane. 



This is in all essential points Wolffs theory. We may omit 

 his statements on the subject of the first formation of leaves at 

 the growing point and of the development of the parts of the 

 flower, as well as his physiological views on food and sexuality, 



