140 



BOTANY. 



resting spore (e), which soon becomes coated with a thick 

 wall (/). After a longer or shorter time the resting spore 

 may germinate, which it does by bursting its wall and di- 

 viding its contents into two parts, each of which finally 

 becomes a new desmid {g, li, i). 



246. The Diatoms {Diatomacece) are microscopic uni- 

 cellular water-plants, resembling the desmids, but differ- 

 ing from them in having walls which are silicified, and 

 in the chlorophyll being hidden by the presence of a 

 yellow coloring matter (phycoxanthin). Each cell is usti- 

 ally composed of two similar portions, called the valves* 



Each valve may be described 

 as a disk whose edge is 

 turned down all around, so 

 as to stand at right angles to 

 the remainder of the surface, 

 making the valve have the 

 general plan of a pill-box 

 cover. The two vajves are 

 generally slightly different in 

 size, so that one slips within 

 the other {A, Fig. 73), thus 

 forming a box with double 

 sides. In other cases the 

 „ „„ , ^ ^ . , valves are simply opposed 



Fig. 72.—^, front view of a dia- ^ •' ^^ 



torn, showing the overlapping walls; and do not overlap. 

 B, same view of a diatom undergo- -^ 



ing fission ; C, side or top view of a oA"? rT'T,„ ;»,^i„;,q,,„i„ «,„„ 

 diatom (Navicula viridis), showing '*47. ihe individuals may 

 markings. Highly magnified. ■ ± ■ -i ■ i „ ■ 



exist Singly or m loose fami- 

 lies; they are free, or attached to other objects by Httle 

 stalks, and they are frequently imbedded in a mucous se- 

 cretion. The free forms are locomotive, and may be seen 

 in constant motion under the microscope : the mechanism 

 of the motion is not certainly known. 



