156 Scientific Proceedings, Royal Dublin Society. 



color, Harv.] I can only conjecture it to be a morbid en- 

 largement of the part from some unknown cause." 



In the summer of 1889, when engaged in the Laboratory of the 

 Marine Biological Association in an investigation of the flora of 

 Plymouth Sound, I was fortunate enough to dredge a scrap of N. 

 versicolor, Harv. Examination of the material suggested to me an 

 explanation of the callosities which I was enabled to test by the 

 discovery in the summer of 1890, of a locality in Plymouth Sound 

 from which material could be obtained at will by dredging in five 

 to seven fathoms.^ 



Structure of Callosity. — On making a microscopic examination 

 of a cross section of a callosity, the cells composing it are seen to be 

 very regularly arranged in vertical rows as though the cells in a 

 row had a common origin (PL xiv., fig. 3) . There may be as many as 

 20 to 30 cells in a row. By examination of suitable material, all 

 stages in the formation of a callosity can be followed, from the 

 primary division into two of the single layer of cells of the thallus. 

 I have not been able to make out any definite layer of cells from 

 which the cells of the rows may take origin. Apparently the thicken- 

 ing takes place from within — «.e,the youngest cells are most internal. 

 Each cell is seen to be full of granular bodies which prove, on 

 treatment with suitable reagents, to be starch grains, and crystal- 

 loids (PI. XIV., fig. 5). The callosities are thus reservoirs of reserve 

 material (carbohydrate and proteid), formed on the thallus by a 

 kind of " secondary thickening." The root of the plant (unknown 

 to Harvey) is simply a slight enlargement of the fleshy-carti- 

 laginous stalks. Examination of a cross section of the stalk shows 

 the same regularity of arrangement of the cells as in a callosity. 

 The cells are, however, comparatively empty ; they have no stored 

 contents. It is important to note, too, that the stalk is not, as 

 described, cylindrical ; it is biconvex in cross section, the two edges 

 are broken, and here and there show traces of the thallus (PL xiv., 

 fig- 6). 



Function of Callosities. — Taking into account the similarity of 

 structure of the callosity and of the stalk, the presence of reserve food 

 products in the cells of the callosity, their absence in the stalk cells, 



1 The dredge should pass clear of the moorings of the Duke Rock buoy, on the 

 west side, in a direction parallel to Staddon Heights. The plants will be found growing 

 on muddy stones generally. 



