244 XIX. STRUCTURE OF THE THALLOPHYTA. 



of cells, and this, in stronger stems, to a not inconsiderable degree. 

 The wings of the thallus gradually die up to the midrib, while the 

 growing cortical layer in the periphery of the stem gradually closes 

 together. 



The Sterile Conceptacles. If the thallus is held up to the light 

 and observed with a lens, we notice a number of, as a rule, 

 irregularly- scattered dots, which are wanting only over the mid- 

 rib. Even with the naked eye these dots appear as protuber- 

 ances. On surface-sections each appears as a round opening, 

 surrounded by a projecting rim, and out of which a tuft of long 

 hairs projects. They are pits, no doubt allied to the conceptacles 

 which we shall study hereafter in considering the sexual pro- 

 cesses, and which we can call sterile conceptacles, If we 

 prepare a cross or longitudinal section through such, it appears as 

 a flask- shaped hollow. The hollow is surrounded by the cells of 

 the inner cortex. From the cells at the base of the hollow arise 

 long threads, composed of elongated cells, which project from the 

 mouth of the sterile conceptacle. It is quite possible that these 

 hairs facilitate the absorption of nutrient materials from the 

 surrounding water. In sections through older parts of the thallus, 

 we can find, between the long hairs in the pits, also bundles of 

 shorter unicellular hairs, which do not reach to the mouth of the 

 pit. If, lastly, still older parts are examined, where the pits 

 present themselves as brown spots, we find the outer parts of the 

 long hairs destroyed, and the opening of the pits closed by s the 

 basal parts of these hairs, by the short hairs, and by a brownish 

 mucilage. 



The Bladders. Sections which are taken through a young 

 bladder or vesicle of the thallus, show its interior filled with a 

 plexus of the same threads which we found in the wings of the 

 thallus. Scattered in the jelly between the threads are bubbles of 

 gas, which sometimes tear the loose tissue and form large air- 

 chambers. The older bladders are quite hollow, filled with air, 

 remnants of the threads of cells being found on the wall. This 

 latter is covered by the outer layer, and shows a thick cortex, 

 arisen from the tangential division of the other cortical cells. 



The Characece. Nitella. The small but widely-distributed 

 family of the Characese (or Stoneworts) occupies a well-nigh 

 .isolated position in the Vegetable System, but can best be in- 

 cluded amongst the green Algae. They are characterised by a 

 Temarkable vegetative structure, and, as we shall see hereafter, 



