xxix APICAL GROWTH 351 



pass insensibly into the permanent tissues, the cells near the 

 surface gradually merging into epidermis and hypodermis, 

 those towards the central region into sclerenchyma and the 

 various constituents of the vascular bundles, and those of 

 the intermediate regions into parenchyma. 



The examination of the growing end of the stem shows us 

 how the process of apical growth is carried on in a compli- 

 cated plant like the fern. The apical cell is continually 

 undergoing fission, forming a succession of segmental cells ; 

 these divide and form the apical meristem, which is thus 

 being constantly added to at the growing end by the formation 

 and subsequent fission of new segmental cells : in this way the 

 apex of the stem is continually growing upwards or forwards. 

 But at the same time the meristem cells farthest from the 

 apex begin to differentiate : some elongate but slightly, 

 increasing greatly in size, and become parenchyma cells : 

 others by elongation in the direction of length of the stem 

 and by thickening and lignification of the cell-wall become 

 sclerenchyma cells : others again elongate greatly, become 

 arranged end to end in longitudinal rows, and, by the loss 

 of their protoplasm and of the transverse partitions between 

 the cells of each row, are converted into vessels spiral or 

 scalariform according to the character of their walls. Thus 

 while the epidermis, parenchyma, and sclerenchyma are 

 formed of cells, the spiral and scalariform vessels are cell- 

 fusions, or more accurately cell-wall-fusions, being formed by 

 the union in a longitudinal series of a greater or less number 

 of cell-walls. It will be remembered that the muscle-plates 

 of Polygordius are proved by the study of development to be 

 cell-fusions (p. 305). 



We thus see that every cell in the stem of the fern was once 

 a cell in the apical meristem, that every vessel has arisen by 

 the concrescence of a number of such cells, and that the 



