i8 



GROWING POINTS 



cell has the form of a three-sided pyramid with a rounded base 

 directed outwards (Fig. 7, a.c.) ; such a cell is described as 

 tetrahedral. Segments are here cut off in succession from each 

 of the three flat inner faces, these segments undergoing further 

 subdivision to form the mature tissues. This type is found in 

 Mosses and Ferns, and can be readily examined in a longitudinal 

 section through the tip of the stem of a Horsetail Fern (Equisetum) 

 (Fig. 7). In such a section the apical cell (a.c.) of course appears 



(XC. 



FIG. 7. Growing apex of the Giant Horsetail (Equisetum maximum) in 

 longitudinal section, showing the tetrahedral apical cells of the stem 

 and leaf (a.c.) ; /, leaf. 



triangular, and two only of the three series of segments are visible. 

 In the roots of Ferns and other Cryptogams which possess such 

 a unicellular growing point its mode of division is similar, but 

 segments are also cut off parallel to the rounded base, the cells 

 produced by their subdivision forming the root-cap. 



The growing point of the higher plants consists of a tissue 

 of small actively growing and dividing elements, which takes 

 the place of the single cell of the lower forms. Such a tissue 

 is called a meristem, and is best examined by cutting longitudinal 

 sections through the apex of stem or root. In both organs the 



