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 



Fig. 141. — 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 diatom in. ' 



Cell-division and auxos pores. — 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 auxosporcs 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 



1 The precise relation of these pigments is not certainly known. 



