2 THE PLANT CELL. 



firm cell-boundary, but is motile and creeps about, and also in- 

 gests food by means of protrusions (pseudopodia) pushed out from 

 its clear outer portion, or ectoplasm (see Chap. i.). 



ii. Cells possessing Definite Cell-walls dnring their whole 

 Existence. — Here a firm limiting membrane is present, and the 

 protoplasm although capable of moving within these limits, 

 cannot move freely from one position to another. Further on, it 

 will nevertheless be seen that " pits '' or perforations may exist in 

 the cell-wall, by means of which the protoplasmic contents of 

 adjacent cells are put into communication with one another ; and 

 at times the protoplasm may pass slowly through these " pits " 

 so as to vacate one cell-cavity for another, leaving its former 

 casing quite empty. 



iii. Motile cells, often possessing no differentiated cell-wall, but 

 the outermost portion of the protoplasm is much firmer than the 

 inner portion, thus forming a more or less resistant boundary. 

 These cells have one or more protrusions of the firm outer 

 protoplasm (ectoplasm), known as cilia, which are active in 

 producing movements of translation or rotation (swarmspores). 



We shall notice more especially the second of these sub- 

 divisions. 



Tissues and their Arrangement : Ftinction : Classification of 

 Plants according to Evolution. — Before proceeding to the detailed 

 study of the various cells of which a plant is made up, it is 

 necessary to examine, briefly, the manner in which cells are 

 grouped into tissues, and the nomenclature, general arrange- 

 ment, and function of these as they occur in such organs of a 

 plant as stem, root, and leaf; and it will also be convenient to 

 have an outline of the main groups and subdivisions into which 

 the vegetable kingdom is divided, from the point of view of 

 evolution. 



In lower plants, such as the Algae, there are often found cells 

 living as single organisms during their whole existence, and yet 

 others are joined together so as to form a colony, such as a 

 filament, or a flat, or round mass of cells, which live together 

 forming what is known as a cell - community. Occasionally 

 plants of a low order are observed, which are to all external 

 appearances somewhat highly differentiated (Fucus, Laminaria), 

 but which, nevertheless, when their internal structure is examined, 

 are found to be of comparatively simple organisation. 



