CHAP, ii.] Phytotomy in the Eighteenth Century. 251 



search out the true nature of vegetable cell-structure, and 

 to explain it on physical and philosophical grounds. The 

 observations themselves on this point are highly inexact, and 

 influenced by preconceived opinions, and his account of them 

 is rendered obscure and often quite intolerable by his eager- 

 ness to give an immediate philosophic explanation of objects 

 which he had only imperfectly examined. His efforts to 

 follow the course of development in the first beginnings of 

 the formation of cell-tissue were evidently not seconded by 

 sufficient knowledge of the structure of matured organs, and, 

 to judge by his figures and by his theoretical reflections, his 

 microscope was of insufficient power and its definition imper- 

 fect. Notwithstanding all these deficiencies, Wolffs treatise is 

 doubtless the most important work on phytotomy that appeared 

 in the period between Grew and Mirbel, not, as has been said, 

 on account of any particular excellence of observation, but 

 because its author was able to make some use of what he saw, 

 and to found a theory upon it. 



According to that theory all the youngest parts of plants, 

 the punctum vegetationis in the stem, which Wolff first 

 distinguished, the youngest leaves and parts of the flower, 

 consist of a transparent gelatinous substance ; this is saturated 

 with nutrient sap, which is secreted at first in very small drops 

 (we might say vacuoles), and these, as they gradually gain in 

 circumference, expand the intermediate substance and so 

 present enlarged cell-spaces. The intermediate substance 

 therefore answers to what we should now call the cell-walls, 

 only these are at first much thicker, and are constantly becom- 

 ing thinner with the growth of the cell-spaces. We may 

 compare young vegetable tissue, formed as Wolff imagines, 

 with the porosity of fermenting dough, except that the pores 

 are not filled with gas but with a fluid. It is plain from the 

 above description that the vesicles or pores, as Wolff names 

 the cells, are connected together from the first by the inter- 

 mediate substance, and that one lamina or cell-membrane 



