POLYTRICHUM OOPHYTE GENERATION 181 



3. The broad band of tissue of the cortex has relatively thin, 

 yellowish or colourless walls, and protoplasmic contents with 

 starch-granules, and globules of oil (compare sections treated 

 with iodine). 



4. The central mass of. tissue, which may be compared to a 

 vascular bundle, is not sharply limited from the cortex : it 

 consists of 



a. A peripheral, small-celled, and thin-walled portion, 



the walls of which do not stain blue, but light yellow 

 with Schulze's solution. 



b. A central, thick-walled part, without cell-contents : the 



thick walls stain dark brown with Schulze's solu- 

 tion : the elements are often divided by delicate 

 septa, which are not stained by Schulze's solution. 

 This tissue may be compared to the xylem of true 

 vascular plants. 



Here and there small groups of tissues similar to the above 

 may be seen in the cortex ; these are the strands 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. 



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 serrated margin : the basal portion of the leaf, which 

 is closely applied to the stem, is broad, but thin and membran- 

 ous, and is not marked by longitudinal striae. 



