300 



THE FORMS OF TISSUES 



[CH. 



radius of the other sphere ; in Uke manner, make the angle 

 c'jyn = 60°, cutting the Hne cc' in c" ; then c' will be the centre 

 of the second sphere, and c" that of the spherical partition. 



Whether the partition be or be not a plane surface, it is obvious 

 that its line of junction with the rest of the system lies in a plane. 



Fig. 105. 



Fig. 106. 



and is at right angles to the axis of symmetry. The actual 

 curvature of the partition- wall is easily seen in optical section ; 

 but in surface view, the line of junction is projected as a plane 

 (Fig. 106), perpendicular to the axis, and this appearance has 

 also helped to lend support and authority to " Sachs's Rule." 



B 



oooocCJcco 

 ooooooo ; 



OOCZ3CIDO. 



Many spRerical cells, such as 

 Protococcus, divide into two equal 

 halves, which are therefore separ- 

 ated by a plane partition. Among 

 the other lower Algae, akin to 

 Protococcus, such as the Nostocs 

 and Oscillatoriae, in which the 

 cells are imbedded in a gelatinous 

 matrix, we find a series of forms 

 such as are represented in Fig. 107. 

 Sometimes the cells are solitary 

 or disunited; sometimes they run 

 in pairs or in rows, separated one 

 from another by flat partitions ; 

 and sometimes the conjoined cells 

 are approximately hemispherical, but at other times each half 

 is more than a hemisphere. These various conditions depend, 



c QGoxECDaio: 



Fig. 107. Filaments, or chains of 

 cells, in various lower Algae. 

 (A) NoHoc; (B) Anabaena; (C) 

 Rivularia; (D) Oscillatorin. 



