332 THALLOPHYTES 
Usually there is also a longitudinal line, which is a fissure or serics 
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- 
lar 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 
Fic. 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 sidc one valve 
is seen to fit over 
the other. From 
Strasburger. 
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 awxospore (meaning en- 
larging spore). 
It-is in connection with 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 zodspores or 
gametes like the Brown Algae. 
Stoneworts. — The Stoneworts constitute the group scientifi- 
cally known as the Charales. 
Some classify the Stoneworts as 
