332 



THALLOPHYTES 



Usually there is also a longitudinal line, which is a fissure or series 

 of fine pores through which fine threads of protoplasm project 

 and serve like cilia in locomotion. The halves of the box-like 

 shell of a Diatom are called valves and the appearance of a 

 Diatom depends much upon whether the face of the valve (the 

 valve side), or the side showing the joining of the valves (the 

 girdle side) is seen {Fig. 286). The protoplast usually has a large 

 central vacuole with the nucleus suspended in 

 the center by small strands of cytoplasm. 



Cell division is the chief method of repro- 

 duction. The cell usually divides lengthwise 

 and in such a way that the valves separate 

 with the daughter protoplasts. Each daughter 

 protoplast then develops a new valve on the 

 naked side. In connection with this, a pecul- 

 iar situation arises. The new valve de- 

 veloped always fits within the old one and 

 consequently there is a gradual reduction in 

 the size of the individuals as division con- 

 tinues, for at each division the daughter 

 protoplast with the smaller valve is necessarily 

 smaller than in the preceding division. How- 

 ever, it has been found that the protoplasts 

 shed their walls when reduction in size has 

 reached a certain degree and in a naked 

 condition grow to full size and then enclose 

 themselves in new valves. This naked pro- 

 toplast is called an auxospore (meaning en- 

 larging spore). 



It is in connection wdth these naked proto- 

 plasts that the sexual act occurs. Sometimes 

 the protoplasts of contiguous cells conjugate 

 and sometimes the four daughter protoplasts of two contiguous 

 cells escape and conjugate in pairs. The zygospore usually 

 enlarges and then encloses itself in valves. 



Thus Diatoms are one-celled and conjugate like some Green 

 Algae, have the color of Brown Algae but have no zoospores or 

 gametes like the Brown Algae. 



Stoneworts. — The Stoneworts constitute the group scientifi- 

 cally known as the Charales. Some classify the Stoneworts as 



Fig. 286. — a 

 common Diatom, 

 Navicula viridis, 

 with valve side 

 shown at the left 

 and the girdle side 

 at the right. In 

 the view of the 

 girdle side one valve 

 is seen to fit over 

 the other. From 

 Strasburger. 



