30.] CHAPTER II. THE TISSUES. 151 



The Growing-point in the lower plants. In the gametophyte of the Bryophyta, 

 the growing-point of the stem or of the thallus has either a single apical cell 

 (all Mosses ; Jungermanniaceae) or a group of apical cells (Marchantiaceae, Antho- 

 cerotaceae) : the growing-point of the sporophyte of the Liverworts has a group 

 of four initial cells, whilst that of the Mosses has a single two-sided apical cell. 

 In none of these plants is there a true dermatogen ; consequently, like the stem 

 of the Pteridophyta, they are destitute of a true epidermis. In both the stem of 

 the gametophyte and the thalloid shoot of the sporophyte of certain Mosses, 

 what may be regarded as a single central stele is differentiated. 



In the higher Algae, the shoot (or thallus) also grows by means of a single 

 apical cell : in the more filamentous forms (e.g. some Florideae, Characese) the 

 apical cell is hemispherical in form, and segments are cut off by transverse walls 

 in one plane only ; in other more bulky forms of Florideae there is a group of 

 initial cells ; in nearly all these forms a more or less distinct differentiation of 

 a central medullary tissue and of a cortical tissue takes place : in the Fucacese 

 there is a single apical cell in the growing-point, with either three or four flanks 

 along which segments are cut off; when the apical cell is four- sided, segments 

 are also cut off internally along the truncate base of the cell ; the tissues soon 

 show differentiation into a cortical and a medullary region. 



In unseptate or imperfectly septate plants, having apical growth (e.g. Siphon- 

 aceous Algae, Cladophora, etc.), the growing-point (like the rest of the body) is 

 not cellular, but consists merely of embryonic protoplasm. 



d. The Morphology of the Stele. The plerome, constituting- the 

 young stele, always gives rise to vascular tissue and usually to a 

 certain amount of fundamental tissue which is distinguished as 

 intra-stelar fundamental tissue, or more briefly as conjunctive tissue. 



The first indication of the development of vascular tissue in 

 the stele is afforded by the differentiation of a varying amount of 

 procambium or primary desmogen, consisting of somewhat elongated 

 narrow cells formed by repeated longitudinal division, which is 

 the embryonic tissue from which the vascular tissue is eventually 

 formed. The procambium frequently constitutes one solid central 

 strand, surrounded by more or less conjunctive tissue constituting 

 the pericycle ; this is sometimes the case in slender roots (see 

 Fig. 131), in slender monostelic stems (e.g. many aquatic Phanero- 

 gams, such as Callitriche, Myriophyllum, Elodea, Naias ; rhizomes 

 of Adoxa, Corallorhiza, Epipogum; among Pteridophyta, Isoetes, 

 Salvinia, Azolla, Psilotum, Lycopodium, Hymenophyllum, Lygod- 

 ium, Gleichenia, Schizgea), and generally in the steles of poly- 

 stelic stems. More commonly, however, the procambium of stout 

 roots and monostelic stems is developed as a number of strands 

 variously arranged in the stele, generally in a circle or in several 

 circles ; the strands thus forming an incomplete hollow cylinder 

 enclosing a central mass of conjunctive tissue, the medulla orpiiJi, 



