LECTURE II. 



THE STRUCTURE AND PROPERTIES OF THE PLANT-CELL. 



FROM the preceding lecture we have learned that the 

 cells of plants vary in their structure: the zoospores of 

 Haematococcus afforded examples of naked or primordial cells 

 consisting simply of protoplasm, and Haematococcus and 

 Saccharomyces of cells possessing a cell-wall. 



An examination of the tissues of a highly organised plant 

 will shew that a third form of cell exists, cells, namely, which 

 consist only of cell-wall. These cells are necessarily dead ; 

 they are merely the skeletons of cells which once contained 

 protoplasm and were living. Still they are not. useless ; they 

 give firmness and rigidity to the plant, and, as we shall see in 

 detail hereafter, they are of importance in connexion with the 

 conduction of water through the plant. 



Since living cells are, in the vast majority of cases, pro- 

 vided with a cell-wall, our study of their structure and 

 properties had better be made upon such as have this com- 

 position. For this purpose the cells of the cortical parenchy- 

 matous tissue of the stem of some succulent plant may be con- 

 veniently taken. When a section of this tissue is examined 

 with the microscope it is seen (Fig. 3) that the cells are 

 separated by cell-walls, that in each cell there is a layer of 

 protoplasm which is in contact with the whole surface of the 

 cell-wall ; that either in this parietal layer or in a central mass 

 of protoplasm which is connected by protoplasmic strands with 

 the parietal layer there is a well-defined somewhat roundish 



