GROUP II. BRYOPHYTA : MUSCI. 355 



contain chloroplastids, and their walls are brown and their septa 

 oblique. The protoriema presents, in fact, a certain differentiation 

 into shoot and root, the term rhizoids being applied to the root-like 

 filaments. This differentiation is, however, of little morphological 

 value, since the differences between the shoot- and root-filaments 

 depend entirely on external conditions : thus, if the rhizoids be 

 exposed to light they assume the characters of the subaerial 

 filaments. In Andresea the differentiation is altogether wanting. 

 The flattened protonema of Sphagnum bears rhizoids on its 

 margins and under surface. 



The growth in length of the protonemal filaments is apical : the 

 terminal cell behaves as an apical cell from which segments are 

 successively cut off by transverse or oblique walls. 



The Adult Shoot arises as a lateral bud on the subaerial portion of 

 the protonema. In some cases (Bryineee) the subterranean portion 

 gives rise to lateral buds : these are small, spherical or lenticular, 

 multicellular bodies of a brown colour, filled with reserve materials, 

 and are termed bulbils- when they are brought to the surface 

 they give rise to adult shoots, either directly or with the interven- 

 tion of protonema. 



The adult shoot is in all cases differentiated into stem and 

 leaves, and may be branched or unbranched. Its symmetry is 

 commonly radial or isobilateral, less commonly dorsiventral. In 

 the former case it is attached to the soil by rhizoids springing 

 from its basal portion ; in the latter, by rhizoids developed on its 

 under surface. In Sphagnum, rhizoids occur only on young 

 shoots. 



The growth of the adult shoot (and its branches) is affected by 

 means of an apical growing-point with a single apical cell which 

 is generally a three-sided pyramid : in Fissidens, however, though 

 the subterranean shoots, and in some species the subaerial lateral 

 shoots, have a three- sided apical cell, yet, under the influence of 

 light, the apical cell eventually becomes two-sided. 



Each segment cut off from the apical cell gives rise to a leaf : 

 hence the arrangement of the leaves, and the symmetry of the 

 shoot, is generally determined by the form of the apical cell. 

 Thus in Fissidens, the leaves are arranged in two rows and the 

 symmetry of the shoot is isobilateral : in other cases (e.g. Fontin- 

 alis) the leaves are in three rows, and the symmetry of the shoot 

 is radial. But to this rule there are exceptions. Thus, in. the 

 sterile shoots of Schistostega, although the apical cell is three- 



