THE CELLULAR CONSTRUCTION OF THE PLANT 19 



to similar scliemes. Thus in the young state the axis, and, it may be 

 said more generally, the plant-body throughout, is partitioned up 

 into cells in somewhat the same way as a house is partitioned into 

 rooms. And their arrangement is not at haphazard, but according 

 to rule. It may be stated generally, as a fact of experience^ that the 

 whole of the plant-body, whether young or mature, is made up of such 

 cells, or their derivatives. This generalisation used to be spoken of 



Fig. 10. 



Diagram illustrating the plan of arrangement of cell-walls in the apex of the stem 

 of an Angiosperm. ATA" = axis of construction. ££ = external surface. PP=peri- 

 clinal curves. ^^ = anticlinal curves. (After Sachs.) 



as the " cellular theory." But it is now so fully demonstrated that 

 the fact may be enunciated as a positive conclusion. It will be seen 

 later what are the modifications which such cells undergo so as to 

 produce the mature tissues of the plant, which often differ widely in 

 form and structure from the young cells that give rise to them. 



From a comparison of cells in various states of division it is possible 

 to construct a connected history of the process (Fig. 11). The nucleus 

 takes the initiative (i.-iv.). By complex changes, which will be 

 described in detail later, it divides into two exactly equivalent parts, 

 which at first lie in the longer axis of the cell, embedded in the still 

 undivided cytoplasm (v.-vii.). Then a delicate film of cell- wall is 

 formed between them, inserted at right angles to the pre-existent walls, 

 cutting the cell into two nearly equal parts, each containing a nucleus 

 (viii.-ix.). Such simple divisions are called somatic, belonging to the 

 soma or plant-body, to distinguish them from certain divisions which 

 involve further comphcations connected with the reproductive process. 

 The number of somatic divisions is indefinite, and the numerous cells 

 to which they give rise are while young thin-walled, and all alike. 



