iLLAN : Observations on Nereocystis 285 



sieve-tubes stood in radiating lines of three or four to each group. 

 Although the trumpet-hyphae, reserving this term for the sieve- 

 tube-like elements of the pith-web in contradistinction to those 



I 



developed from the cortex, were not yet apparent in my 12 cm. 

 material there were some perforated transverse walls visible in the 

 ordinary anastomosing filaments of the web. Although my stains 

 have not been nuclear some excellent mitotic figures were visible 



in this preparation. - 



Longitudinal sections through the 12 cm. material show the 

 cortex to consist in general of thin-wallcd parenchymatous tissue 

 made up of cells about twice as long as broad in the layers close 

 to the epidermis but becoming progressively longer and slenderer 



■ 



towards the central cylinder, until those cells bounding the pith- 

 web become transformed Into the sieve-tubes. The pith-web cells 

 are attached to the sieve-tube cells and to the undifferentiated lay- 

 ers just outside of the sieve-tube zone. The young sieve -tube is 

 indistinguishable from an ordinary prosenchymatous cell of the 

 Inner cortex. The nuclei of some inner cortex cells undergo 

 fragmentation and then these cells are greatly elongated as the 

 stipe grows in length. As one elongates it becomes much nar- 

 rower, so that while the diameter of the cell from which a sieve- 

 tube originates may be 12 mic. and the length 80 mic. the diame- 

 ter of the sieve-tube which arises from it may in Its thinnest portion 

 be scarcely more than i mic. while the length may exceed a 

 millimeter ! Cross sections through the slenderest part of such a 

 sieve-tube show its wall to be thickened like a thermometer tube, 



■ 



while the cell contents, deeply stainable with aniline blue, fill the 

 extremely delicate capillary cavity. 



Observation of a series of longitudinal sections makes it seem 

 probable that the first sieve-tubes formed, and many of the suc- 

 cessive tubes, are elongated to such tenuity that they finally pull 

 apart in the middle and then the free ends of the tube deliquesce 

 into the common gelatinous slime of the pith-w^eb leaving only the 

 thickened so-called callus patches attached to the remnants of the 

 "tube and even these may disappear. The centrifugal production 

 of siev^e-tubes continues vigorously while the stipe is young, but in 

 old material not so large a number proportionately are to be found. 

 The stretching of the sieve-tube has all the appearance of a passive 



