CHAPTER I. 

 THE CELLS AND TISSUES OF PLANTS. 



A. THE NATURE AND SIGNIFICANCE OF CELLULAR STRUCTURE. 



Every discussion of the internal structure of plants, whether it 

 be morphological or physiological in tendency, must be based upon a 

 clear conception of the nature of vegetable cells. On this account 

 the present chapter must be prefaced by a statement of certain 

 elementary facts, involving no theoretical assumptions, concerning 

 cellular structure. 



If any filamentous Alga such as Oedogonium or Spirogyra be 

 examined microscopically, it will be found that the liquid and semi- 

 liquid constituents of the plant-body are enclosed within a thin-walled 

 but relatively firm tube. This tube is however not traversed by a 

 continuous cavity, but is at certain intervals divided into separate 

 chambers or compartments by transverse septa consisting of the same 

 substance as the outer wall of the tube. Each of these compartments 

 encloses a separate portion of the above-mentioned liquid and semi- 

 liquid contents. The filament is thus made up of a series of well-defined 

 compartments, which may be regarded as the structural units or 

 elements of the plant-body. Each unit is termed a cell, and the 

 thallus of such an Alga is described as a cellular filament. 



If a simple type of Moss-leaf be next examined from a similar 

 ] ii iiut of view, it is at once evident that in this case the structural units 

 are disposed in two dimensions of space, or, in other words, that such 

 a leaf represents a cell-plate. Viewed from above, the cell-walls 

 present the appearance of a delicate network, the different meshes or 

 compartments of which vary greatly in shape and size. About the 

 middle of the leaf the meshes are elongated, on either side thereof 

 the cells are more polygonal in outline, while the leaf margin again 

 is mainly occupied by elongated cells with tooth-like projections 

 directed towards the outside. This variety of shape exhibited by the 



