4iS FERNS LESS. 



small cells, the inner called the phloem-sheath or pericyde, the 

 outer, the bundle-sheath or endodermis (b. sh\ 



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

 matous, but among them are some to which special 

 attention must be drawn. These (B and c, sv. t\ are many 

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

 irregular patches or sieve-plates (F, sv.pl.) composed of 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 a non- 

 cellular tissue as in the deric epithelium and certain other 

 tissues of Polygordius (see p. 289). 



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 meristem (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. 1 08, H, 

 p. 401), 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 



