XXXI APICAL GROWTH 419 



of small, dose-set cells without intercellular spaces. As the 

 base of the apical cone is reached, the meristem is found to 

 pass insensibly into the permanent tissues, the cells near the 

 surface gradually mergmg 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 forma- 

 tion 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 difierentiate : 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- 

 jusions, 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. 302). 



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



E K 2 



