432 A TEXTBOOK OF THEORETICAL BOTANY 



Below the leafy region of the Polytrichum stem the anatomy changes 

 and we enter the portion known as the rhizome. The outline of the stem 

 is here broadly triangular and the cortex is divided by three radial strands, 

 which penetrate into the central cylinder, giving it a three-lobed outline. 



leptome 



Leaf trace 



Hadrome 



Fig. 419. — Polytrichum commune. Transverse section 

 of aerial stem showing central conducting strand 

 and leaf traces in cortex. 



The outer tissues consist of three to four layers of cortex, the outermost 

 layer of which is strongly suberized. The innermost layer consists of very 

 large cells with thin suberized walls, known as the endodermis. Within 

 this is the central cylinder, surrounded by a layer of thin-walled cells which 

 represent a primitive pericycle. The greater part of the central cylinder is 

 made up of thick-walled, somewhat elongated cells, forming the sterome, 

 among which are scattered groups of empty, elongated cells, the hydroids. 



The three radial strands start from groups of thick-walled cortical cells, 

 while their inward ends, embedded in the central cylinder, consist of thin- 

 walled leptoids, corresponding to the leptome in the upper stem. This 

 inner group of leptoids is separated from the tissue of the central cylinder 

 by a single layer of small cells containing starch, called the amylome. The 

 triradiate structure of the central cylinder, with three separate leptome 

 strands, affords a striking analogy to the alternating arrangement of vascular 

 tissue in the root structure of higher plants, which bears the same relation 

 to the concentric arrangement of the stem, that the rhizome of Polytrichum 

 does to the concentric arrangement of tissues in its upper stem. 



In Polytrichum all these structures are extremely primitive (Fig. 420) 

 compared with the conducting systems of the higher vascular plants. We 

 must bear in mind that they are analogous, but not homologous, w^ith those 



