Dec, 1910.] Self-Dividing Laminae of Certain Kelps. 219 



followed; paraffine fomiing the embedding medium and the sec- 

 tions eut 10 mic. thick. The single stain aniline safranin or the 

 same in combination with gentian violet w^ere used. The first 

 stain gives the middle lamella of gelatinous interlacing food con- 

 ducting hyphae a characteristic tint which is of much value in dis- 

 tinguishing it from the adjacent cortex. All drawings were made 

 with the camera lueida. 



To understand the splitting of the kelp lamina and its relation 

 to the tissues through which it passes, a digression must be made 

 to set forth the manner of growth in the kelps, with special con- 

 sideration of the derivation of the tissues. Three systems of 

 tissues make up the kelp thallus: the epidermis, the underlying 

 cortex and the central pith-web. Sections of stipe or lamina 

 show the hypha-like elements of the pith-web to be highly stretched 

 and modified cortex cells and the cortex cells are clearly seen to be 

 derived from the epidennal cells, which form therefore the meristem 

 in these plants. By periclinal walls the epidermal cells build the 

 cortex; by anticlinal ones the epidermal area is enlarged. Hypo- 

 dermal and outer cortical cells are often seen dividing, but the 

 total meristematic activity of these internal cells is not nearly so 

 great as that of the epidennal cells. The cells pushed inward 

 from the epidennis reach their maximum size in the middle cortex. 

 On the outside of this expanding cortical zone, the epidemiis cor- 

 respondingly enlarges its area by a constant increase in the number 

 of its relatively smaller cells; the division walls of course being 

 anticlinal. On the inner side of the expanding cortical zone the 

 passive pith-web is seen to consist of much elongated cortical cells 

 (tnimpet hyphae) between which are large intercellular spaces 

 filled with a gelatinous matrix. By this method the large and com- 

 plex kelp thallus originates and the various structures peculiar 

 to the several genera, including the method of branching tmder 

 discussion, have their origin in variations of this simple process. 



NEREOCYSTIS. 



Nereocystis with its prominent splitting line extending far 

 in advance of the cleft, forms an especially favorable type for 

 study as portions of the lamina through which the line passed 

 could be successively investigated down to the actual fissure and 

 the various stages of the process clearly observed. Fig. 2 shows 

 a section through the splitting line at a point corresponding to 

 Fig. 1, a. The changes from nomial lamina are at once seen to be 

 a diminution in the thickness of cortex and pith-web, resulting 

 externally in the fonnation of a broad furrow on each side. A 

 comparison of the affected region with that of nomial lamina at 

 either side, discloses the fact that in the middle region the ratio- 

 of periclinal divisions to anticlinal ones has increased as is evi- 



