LECTURE VI. 



ON THE CELLULAR STRUCTURE OF PLANTS. PROTOPLASM. 

 NUCLEUS. CELL-WALL. 



That plants consist of cells is now known to every well-informed man; 

 yet the true meaning of the word Cell may be quite clear to but few, the less so 

 since biologists themselves, even now, hold and discuss the most different opinions 

 upon it. To many, the cell is always an independent living being, which sometimes 

 exists for itself alone, and sometimes 'becomes joined with' others — milliqns of its like, 

 in order to form a cell-colony, or, as Hackel has named it for the plant particularly, 

 a cell-republic. To others again, to whom the author of this book also belongs, 

 cell-formation is a phenomenon very general, it is true, in organic life, but still only 

 of secondary significance j at all events, it is merely one of the numerous expressions 

 of the formative forces which reside in all matter, in the highest degree, however, in 

 organic substance. 



Such being the case, it is certainly best to leave every theory aside for the 

 present, and confine ourselves to the most immediate experience, and to the consider- 

 ation of a few objects which, without any controversy, consist of cells. 



Let lis first turn for this purpose to Fig. 71, which represents the transverse 

 section of the flower-stalk of a plant closely allied to our common kitchen Onion. 

 We see. at once that its area is occupied by a mesh-work, or is divided into numerous 

 closed chambers. The walls of the latter consist, as is perceived at once, of solid, 

 and, as further experience will show, even of extraordinarily solid substance : the 

 spaces bounded by these contain fluid, and, as we shall see later, other constituents 

 still more important. The size of the chambers is very various, and so is their 

 form. From our figure it is at once observed, however, that the chambers re- 

 sembling one another in size and form are aggregated in layers or groups, arranged 

 on the transverse section of the organ of the plant. 



The individual mesh-like spaces or chambers are the cells: the groups of 

 similar cells are the various forms of tissue. 



An equally thin longitudinal section through the same flower stalk would present 

 a less evident, and apparently less orderly picture. Carefiil consideration would, 

 however, even on the longitudinal section, again enable us to recognise chambers 

 closed on all sides, and bounded by 3. net-work of solid walls; only in this case the 



