DIFFERENTIATION OF TISSUES 



291 



shafts, as in the palm, rather than much elongated cones. 

 It also means lack of ability to develop an extending branch 

 system or to display more numerous leaves each year. The 

 palm may be taken as a typical result of such a structure, 

 with its columnar and unbranched trunk, and its foliage 

 crown containing about the same number of leaves each year. 



The lack of regular arrangement of the bundles also 

 prevents the outlining of a pith region or the organization 

 of definite pith rays. The failure to increase in diameter 

 also precludes the necessity, of bark, with its protective cork 

 constantly renewed, and its sloughing-off phloem. 



To sum up, the stems of the Monocotyledons are 

 characterized by the vascular bundles not developing a 

 cylinder or any regular arrangement, and by collateral and 

 closed bundles, which do not permit increase in diameter, 

 or a branch system, or increase in leaf display. 



153. Pteridophyte stems.— The stems of Pteridophytes 

 are quite different from those of Spermatophytes. While 

 the large Club -mosses (Lyco- 

 2)0clium) and Isoetes usually 

 have an apical group of meris- 

 tem cells, as among the Seed- 

 plants, the smaller Club-mosses 

 (Selac/ineUa), Ferns, and Horse- 

 tails usually have a single api- 

 cal cell, whose divisions give 

 rise to all the cells of the stem. 

 Generally also a dermatogen is 

 not organized, and in such 

 cases there is no true epidermis, 

 the cortex developing the ex- 

 ternal protective tissue. In the cortex there is usually an 

 extensive development of stereome, in the form of scleren- 

 chyma (Fig. 272), the stele furnishing little or none, and 

 the vascular bundles not adding much to the rigidity, as 

 they do in the Seed-plants. 



Fig. 272. Diagram of tissues in cross- 

 section of stem of a fern ( Pfe7is), 

 showing two masses of scleren- 

 chyma {st), between and about 

 which are vascular bundles. — 

 Chamberlain. 



