350 FERNS LESS. 



The cells of the phloem are for the most part parenchy- 

 matous, but amongst them are some to which special 

 attention must be drawn. These (B and c, sv. /), are many 

 times as long as they are broad, and have on their walls 

 irregular patches or sieve-plates (F, sv. pi.} composed f groups 

 of minute holes through which the protoplasm of the cell is 

 continuous with that of an adjacent cell. The transverse or 

 oblique partitions between the cells of a longitudinal series 

 are also perforated, so that a row of such cells forms a sieve- 

 tube in which the protoplasm is continuous from end to end. 

 We have here, therefore, as striking an instance of proto- 

 plasmic continuity as in the deric epithelium and certain other 

 tissues of Polygordius (see p. 276). 



The distal or growing end of the stem terminates in a blunt 

 apical cone or punctum vegetationis (c), surrounded by the 

 leaves of the terminal bud in the case of vertical stems, or 

 sunk in a depression and protected by close-set hairs in the 

 underground stem of the bracken. A rough longitudinal 

 section shows that, at a short distance from the apical cone, 

 the various tissues of the stem epidermis, parenchyma, 

 sclerenchyma, and vascular bundles merge insensibly into 

 a whitish substance, resembling parenchyma to the naked 

 eye, and called apical merisiem (ap. mer). 



Thin sections show that the summit of the apical cone is 

 occupied by a wedge-shaped apical cell (ap. c) which in 

 vertical stems is three-sided like that of mosses (Fig. 80, H, 

 p. 335), while in the horizontal stem of Pteris it is two-sided. 

 As in mosses, segmental cells (seg. c} are cut off from the three 

 (or two) sides of the apical cell in succession, and by further 

 division form the apical meristem (ap. mer\ which consists 

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

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



