THALLOPHYTES 53 
In many cases the valves are sculptured with fine transverse lines (really rows 
of dots), the markings being so regular and minute as to serve as a test of the defini 
tion of lenses. There is also a longitudinal line (raphe), which represents a cleft 
or a series of openings through which pseudopodia are thrust for locomotion. There 
are two distinct aspects of such cells : that showing the relation of the valves being 
the girdle side and that showing the face of a valve being the valve side. 
Structure. The protoplast consists of usually peripheral cytoplasm, a central 
nucleus swung in a bridge of cytoplasm, and two large or numerous smaller brownish 
yellow chromoplasts, often called endochrome plates. Their color is due to the 
FlG. I4i.-Sargassum: showing the appearance of stem and leaves, 
and the berry-like floats. 
chlorophyll (at least a green constituent) and a characteristic golden brown con- 
stituent called diatomin. l 
Cell-division and auxospores. In cell-division the growth of the protoplast 
separates the valves, and division occurring in the plane of the valves, each new 
protoplast possesses one of the old valves and forms a new valve on the naked 
side. It is evident that one of the two cells thus produced has the same size as the 
parent cell ; while the other cell is smaller, for the smaller valve of the parent cell 
becomes the larger (overlapping) valve of the daughter cell. This means that 
among the progeny cells there are series of individuals of diminishing size. When 
this diminution in size has reached a minimum, the auxospores are formed, the 
name indicating " enlarging spores." Auxospores may be produced in a variety 
of ways, the simplest being the separation of the valves and the escape of the proto- 
plast, which in this free condition grows to the maximum size and develops new 
valves. In this case the auxospore is simply the escaped protoplast. In other 
'The precise relation of these pigments is not certainly known. 
