204 PKOCEEDINGS OP THE AMERICAN ACADEMY 



arranged in radial rows. In a longitudinal section of the stipe the 

 cells of the outer cortex are short and more or less irregularly placed, 

 while the cells of the inner cortex are much elongated and in vertical 

 rows. In the blade there are only a couple of layers of cells in each 

 cortex, but in the stipe there are several layers of cells in each, and 

 the cells of the layers are well supplied with pits. 



The pits among the Laminariaeece are very conspicuous structures, 

 and deserve esi)ecial study. They are mentioned and described by a 

 number of writers on the subject, but no satisfactory account of their 

 development has yet been published. They appear quite early in the 

 second period of growth, but are not readily seen until reagents are 

 applied. When the section, however, is treated with chloriodide of 

 zinc solution, or dilute Boehmer's or Kleinenberg's hasmatoxylin, 

 they appear as circular uncolored places, and are large in proportion 

 to the size of the cells on the walls of which they occur. They are 

 to be found for the most part on the walls of the cells of the inner 

 cortex and medulla, but are present sparingly on the walls of the 

 innermost layers of cells of the outer cortex. In the inner cortex of 

 older specimens the cell walls are very much thickened. There is an 

 appearance like the middle lamella of wood cells, and the walls them- 

 selves are plainly stratified. The pit appears to be situated in the 

 stratified portion, and to consist of two opposite depressions in the 

 adjacent walls of neighboring cells, wliich are separated at this point 

 only by a very thin membrane. When treated with chloriodide of 

 zinc or iodine followed by sulphuric acid, the inner lininjis of the cells 

 turn rapidly blue, and after a time there is a faint blue coloration 

 through the whole thickness between two cells. The whole thickness, 

 too, stains deeply with hcematoxylin. 



c. Medulla. — The tubes (or long tubular cells) are a conspicuous 

 feature in sections of the adult plant, both of blade and of stipe. As 

 we have seen, they arise very early in the history of the plant. It is 

 somewhat obscure as to just how the first tube arises. Probably a 

 middle cell is cut off between the two layers, and has a chance to 

 elongate before another one is formed ; for, as we have seen, the 

 tubes in the young plant were few and scattered. Later, however, new 

 tubes are formed from the inner cells of the inner cortex, and come to 

 lie with the old ones in the medulla. The walls of these tubular cells 

 begin to thicken before those of the other cells, and are of consider- 

 able thickness even in the preceding period. These cells form a laj'er 

 adjoining those of the inner cortex on either side in the blade, and in 

 the stipe this layer appears in cross-section as an ellipse of occasion- 



