THE SIMPLER ORGANISMS 61 



upon the stage, and examining the delicate filaments with 

 low power of the microscope, he will at once observe that 

 they are not all alike : different species of Spirogyra often 

 grow together, but the filaments of a single species differ: 

 some are of a richer green, with the chlorophyl bands 

 adjusted closer together about the inner walls of the fibre. 

 Let him select for study a filament with the green 

 bands as far apart as possible (so that the internal parts 

 may not be hidden) and examine it as to the arrangement 

 of its parts. 



The cell. It will be at once apparent that the plant body 

 is composed of elongate cylindric cells placed together end to 

 end : the filament is a simple linear aggregate of cells. Look- 

 ing at a single cell, it will be seen to have a rather thick cell 

 wall squarely cut at the ends. The chlorophyl is restricted 

 to the spiral band, which is not continuous from cell to cell, 

 and which varies considerably in appearance and in number 

 of turns in the cells of different filaments. Focusing upon the 

 upper surface of the cell, the chlorophyl band will be seen 

 most clearly a beautiful wavy band of green, marked with 

 a narrow median ridge, and studded here and there along 

 the course of this ridge with round bodies containing the 

 pyrenoids. Focusing downward, the band appears below, in- 

 clined in the opposite direction, and less clear because of the 

 parts now intervening. Focusing upon the axis of the cell, 

 and looking between the green bands for the more funda- 

 mental parts, there will be seen (and, readily, when one be- 

 gins to see) at the center of each cell a mass of protoplasm 

 containing the nucleus. As compared with the size of the 

 cell, the amount of cytoplasm is small. It consists in: i) 

 the central mass containing the nucleus, 2) slender strands 

 radiating outward therefrom to various parts of the cell, 

 but chiefly to the pyrenoids, and 3) a thin film next the cell 

 wall. This last fits the cell wall so closely and is so trans- 

 parent it is hard to see. It may be drawn into view by 



