41 8 A TEXTBOOK OF THEORETICAL BOTANY 



plants this inner layer of the cortex is showing some specialization. The 

 cells of the cortex become progressively smaller and thicker-walled towards 

 the exterior, and on the outside there are one or two layers of very small 

 strongly thickened cells which function as an epidermis. 



On the lower part of the stem arise long, brown-walled, repeatedly 

 branched filaments of cells termed rhizoids. The cell partitions in these 

 rhizoids are oblique, and close to these partitions branches arise, which 

 may branch again and again in the same way. It is interesting to note that 

 there is a considerable similarity between the structure of these rhizoids 

 and the protonema, which we shall consider later. This close similarity 

 is further exhibited by the fact that rhizoids if kept damp and exposed to 

 light are capable of producing protonemal branches from which fresh plants 

 can arise. 



Here and there in the cortex small isolated groups of thin-walled cells 

 occur, which in coloration and general structure resemble the elements of 

 the central cylinder. We may regard this central cylinder as a very primitive 

 type of conducting system, by means of which material in a liquid form can 

 be transferred from one part of the plant to another, and these isolated 

 bundles of cells lying in the cortex may be regarded as very simple foliar 

 bundles or leaf traces. They are continuous with the midribs of the leaves, 

 but they do not connect with the central cylinder and end blindly in the 

 cortex. Although in Mnium these foliar bundles do not join the central 

 cylinder, we shall find in the more specialized type, Polytrichum, that such 

 continuity exists. 



The apex of the stem possesses a single large apical cell, which is shaped 

 like a three-sided pyramid with the base uppermost. This cell cuts off 

 daughter cells from each of its three sides in turn, the nucleus rotating, so 

 that the plane of nuclear division is parallel to each side successively. Each 

 daughter cell, by division, builds up a segment of new stem tissue, and from 

 each such segment a leaf develops. The leaves thus form a close spiral 

 succession, each leaf being separated from its neighbours in the spiral by 

 one-third of the stem's circumference. This arrangement is described as a 

 one-third phyllotaxy (see Chapter XXII). 



Lateral buds are developed from the base of the apical segment which 

 forms a leaf, though not from all of them. Torsion of the stem during 

 growth carries these buds sideways so that they are never directly under the 

 leaf formed on the same segment, but usually appear as if axillary to one of 

 the lower leaves. These buds may either remain dormant or may develop 

 into side branches. 



The leaf is narrow and pointed, measuring about i mm. by 5 mm. It 

 consists of a central midrib which terminates a little below the point, and a 

 single-layered lamina on either side of it (Fig. 401). The cells of the 

 midrib are very elongated and thick-walled. The cells of the lamina are 

 parenchymatous, whilst those along the edge of the leaf are like those of the 

 midrib, elongated and strongly thickened. These outer cells bear, at 

 approximately equal distances, pairs of sharply tapering teeth, formed of 



