344 PRACTICAL BOTANY 



which enter the stem from the leaves, and pass inwards 

 towards the central bundle. 



III. Cut median longitudinal sections of the mature 

 stem of Polytrichum : mount as before, and note that 

 the peripheral sclerenchyma consists of elongated 

 prosenchymatous elements, while the cells of the 

 massive cortex are of a parenchymatous form. The 

 elements of the central strand are elongated, and their 

 lateral walls smooth, without pits : they are separated 

 one from another by thin oblique septa. Observe that 

 where the median plane of a leaf has been cut through 

 longitudinally, a strand of tissues similar to those 

 constituting the central strand may be traced, passing 

 obliquely through the cortex towards the central strand. 



By cutting transverse and longitudinal sections of the apex of 

 the stem of Polytrichum^ or Fwwria, it may be ascertained that 

 there is in each case a single apical cell of tetrahedral form, that 

 segments are cut off successively from the three sides, and that 

 one leaf originates from each segment : in the transverse sections 

 it may, however, he seen that in both plants the angle of diver- 

 gence between the successive leaves (and similarly between the 

 successive segments) is larger than one-third : thus the leaves 

 form three parastichies, and this will account for the apparent 

 complexity of their arrangement in these plants. In Fissldem 

 the arrangement is in two longitudinal rows or orthostichies, and 

 there is a bilateral wedge-shaped apical cell. In Fontinalis there 

 is a tetrahedral apical cell, but the divergence of both segments 

 and leaves is one-third. 



IV. Strip off a few mature leaves : mount one of 

 them in water, with the upper surface uppermost, and 

 observe under a low power that the narrow, linear 

 upper portion is marked on its upper surface by 

 longitudinal striae (the lamellae), and has a minutely 



