284 MacIMillax : Observations on Nereocvstis 



tere is still definitely differentiated as such and consists of the 

 characteristic flattened cells with thinner walls than those of the 

 fundamental tissue. The central fundamental tissue making up 

 the great bulk of the organ is composed of rather thick-walled 

 approximately isodiametrical parenchyma. The fixation-area con- 

 sists of much smaller thick-walled cells shaped like those of the 



cambium. 



Structure of Stipe. — In a young plant i8 mm. in length the 

 stipe was .25 mm. in diameter midway between the primitive disc 

 and the laminar expansion. Cross sections and vertical sections 

 showed a central cylinder .08 mm. in diameter already sharply 

 differentiated. It could be recognized in cross section by the 

 smaller diameter of the cells and in longitudinal section by tlie 

 greater comparative length. All the cells, however, within the 

 clearly marked epidermal layer could be described as thin-walled 

 parenchyma or prosenchyma. In material of this age mucilage- 

 ducts were not seen, nor had the intercellular spaces of the ^\\\\- 

 web begun to originate. The whole stipe was solid and approxi- 

 mately homogeneous. The cells throughout were strongly stained 

 and many of the nuclei showed sharp and distinct mitotic figures. 



In slightly older material than that just described the first in- 

 tercellular spaces begin to appear as clefts between the lateral 

 walls of the central pith cells, and a little later the layer of cells 

 immediately surrounding the primitive pith develops the first sieve 

 tubes, while at the same time the cortex rapidly increases in thick- 

 ness. In a plant 12 cm. in length the stipe was i mm. in diam- 

 ctcr and the central cyUnder measured ,25 mm. across. In this 

 stage the pith-web made up of loose, anastomosing, branching, 

 septate filaments is well established. The filaments of the pith- 

 w^eb at this time measured about 10 mic. in diameter. Between 

 the meshes of the loose network an abundance of slime was pres- 

 ent. At the periphery of the pith-web lay the ''sieve-tubes/' as 

 they have been termed, the development of which from ordinary 

 cortical cells was not difficult to follow. Most of the centrally 

 disposed siev^c -tubes appeared very much smaller and thicker 

 walled in the cross section than did the peripheral elements. This 

 was due to the longitudinal stretching of the inner and older 

 tubes. In most of the cross sections through this material the 



