82 MERISTEMATIC SYSTEM 



Musci and Equisetaceae, as well as in the bulk of Filicales, the apical 

 cell divides by three sets of oblique walls : its shape is therefore 

 approximately that of a three-sided pyramid, with a convex base which 

 faces outwards. Division takes place in the three planes parallel to 

 the sides of the pyramid in strict rotation, and the resulting segments 

 consequently form a continuous spiral series. Since the outer wall of 

 each segment occupies about one-third of the circumference of the 

 growing-point [as measured in a horizontal plane, of course] the seg- 

 ments appear to be arranged in three vertical rows. These segments 

 soon undergo further divisions. In Equisetum (Fig. 13) the first of 



Pig. 13. 



Stem-apex of an Equisetum. A. Longitudinal section. B. Apical cell, together 

 with the most recently formed segments, as seen from above ; 1-7, the successive 

 primary segment-walls. After Niigeli and Schwendener (from Nageli and Schwen- 

 dener, Das Mikroskop). 



the secondary walls is parallel to the primary segment-wall ; the next 

 wall to appear is radial and divides the segment into two homologous 

 halves. After this stage the tracing out of the successive divisions 

 already becomes a matter of difficulty. The roots of Equisetum, as 

 well as those of Ferns and certain Monocotyledons, also grow by means 

 of a three-sided apical cell. Here, however, the segmentation is 

 complicated as ISageli and Leitgeb first showed by the appearance 

 of transverse divisions. The ordinary oblique walls cut off segments 

 which give rise to the bulk of the organ, just as in the case of the 

 stem. The transverse divisions are responsible for the formation of 

 the peculiar protective structure known as the root-cap : each segment 

 cut off by a transverse wall forms a sort of hood over the apical cell, 

 and the successive hoods piled one upon the other constitute the root- 

 cap (Fig. 14). The order of segmentation is usually such that a single 

 transverse division alternates with three oblique divisions, each of the 

 latter being in a different plane. 



A four-sided (pyramidal) apical cell occurs in the seedling axis and 

 in the- young rhizophores of certain species of Selaginella (e.g. S. Mar- 

 tensii). 



Even in one and the same growing-point the apical cell need not 

 adhere permanently to a single mode of division. Thus Treub has 



