APICAL CELLS 



s:j 



shown that the apical cell in the lateral shoots of Selaginella 

 Martcitsii is at first four-sided, but subsequently becomes three- or 

 even two sided ; the two latter forms of apical cells may further 

 replace one other quite arbitrarily in older apices, the appearance 

 of an oblique segment- wall sufficing to convert a two-sided into a 

 three-sided cell. Goebel states, with reference to the origin of stems 

 from the meristematic leaf-apices of Adiantinu Edgeworthii, that the 

 two-sided apical cell of the leaf becomes directly converted, by an 

 appropriate division, into the three-sided apical cell of a stem. 

 Similarly, the transformation of a root-apex into the growing-point of a 



Fig. 14. 



Longitudinal section through the end of the root of Ptevii hastata. v, apical cell; 

 <-, the successive hood-layers of the root-cap; o-o, protoderm. After Niigeli and 

 Leitgeb (from Sachs, Ted- Book- of Botany). 



stem which takes place in certain Ferns {e.g. Asplenium esculentum and 

 Platycerium alticorne) is initiated, according to Rostowzew, by the 

 omission of the outer or root-cap-forming segments from the segmenta- 

 tion-cycle of the three-sided apical cell. Even among the Pteridophyta, 

 therefore, traces may be found of a condition which is much more 

 prevalent among Phanerogams, that is, the comparative inconstancy 

 exhibited by one and the same plant, or 'even by a single organ, in 

 respect of its mode of growth and as regards the form and arrangement 

 of its apical cells. 



C. GROWTH BY MEANS OF SEVERAL APICAL CELLS. 38 



In view of the accepted definition of an apical cell, it is self- 

 evident that if any given cell of the primordial meristem is to rank 

 indisputably as an apical initial it must fulfil the following two con- 

 ditions : it must retain its original position with reference to the axis 

 of the apical cone, and it must divide actively, so long as apical growth 

 continues. There are three conceivable modes in which several initials 

 may be associated, so that all conform to the aforesaid conditions. 



