242 XIX. STRUCTURE OF THE THALLOPHYTA. 



ments with wide cavities, which in cross-section appear approxi- 

 mately polygonal, but, as the longitudinal section shows, gradually 

 increase in length as we pass farther into the thallus. They 

 contain larger chromatophores, which however are not so crowded 

 as in the outer layer, and therefore can be more readily observed. 

 We include these cells, together with the outer layer, as cortex. 

 The cortical cells are connected together by pits with a porous 

 closing membrane. The innermost layer of the cortical cells bears 

 to the midrib the character of a thickening layer, which com- 

 mences its activity at some little distance from the growing point. 

 By surf ace- sections below the cortex, and by suitable longi- 

 tudinal sections, we determine that the interior of the midrib 

 consists of a tissue of longitudinally-elongated cells, running 

 parallel to one another and to the long axis of the frond, and 

 which are connected together into threads. Laterally, these cells 

 communicate by broad pits, and in part also by short prolonga- 

 tions. In the longitudinal direction, the cells of the threads are 

 only separated by delicate partition walls, which are clearly per- 

 forated in a sieve-like fashion. A similar structure is shown by 

 the partition walls within the lateral prolongations, and by the 

 closing membranes of the pits. The threads are separated laterally 

 by an intercellular jelly, which has arisen from the swelling of the 

 middle lamellae and outer thickening layers of the longitudinal 

 walls of the cells. The medulla of the midrib passes over laterally 

 into the " packing tissue " of the two wings, and this appears as a 

 looser network of irregularly-disposed cell- threads, constructed 

 much as in the pith. Here also the partition walls in the cell- 

 threads and the lateral connections are thin, and with sieve-like 

 interruptions, while the longitudinal walls have produced a still 

 more abundant jelly than in the midrib. 



Cell-contents and Reactions. The contents of the cells of 

 the medulla and of the packing tissue are poor in chromatophores, 

 but on the other hand are often rich in highly refractive granules, 

 which cannot be removed by alcohol, but can by ether, which 

 become brown in osmic acid, and therefore are recognised as 

 fat oil. In each cell a nucleus can be made out. Not infrequently 

 the protoplasmic cell-contents take on a chambered structure in 

 the water in which we investigate them. With the addition of 

 iodine, the cell-contents, excluding the oil-drops, colour yellow- 

 brown, and the nuclei are usually easily visible ; a starch reaction 

 is nowhere found, and the oil here no doubt replaces the missing 



