130 THE MICROSCOPIST. 



92). The walls of vegetable-cells are often thickened by 

 deposit. If this is in isolated patches, the cells are called 

 dotted (Plate VI, Fig. 93), and it is sometimes difficult to 

 distinguish them from porous cells. Many cells have a 

 spiral fibre (Plate VI, Fig. 94), which appears to have 

 been detached from the outer membrane. In the seeds of 

 Collomia, etc., the cell- wall is less consolidated than the 

 deposit, so that on softening the cells by water, the spiral 

 fibres suddenly spring out, making a beautiful object for 

 a half-inch object-glass (Plate VI, Fig. 95). 



The tendency of formed material to arrange itself in a 

 spiral is seen in the endochrome of many of the simpler 

 plants, as Zygnema,a,n& the cell-wall sometimes tears most 

 readily in a spiral direction. 



If the spiral deposit is broken and coalesces at some of 

 its turns, it forms an annulus or ring. Some cells show 

 both rings and spirals. 



For the production of a spiral movement or growth, 

 another force is needed in addition to the centripetal and 

 centrifugal forces which are necessary for curvilinear mo- 

 tion. The centripetal point must be carried forward in 

 space by a progressive force. When we consider that a 

 spiral form is so frequently seen in morphology, that the 

 secondary planets move in spirals round their primaries, 

 and that even in distant nebulae the same law prevails, 

 we are struck with the unity of plan which is exhibited 

 throughout the universe, and can scarcely ail to observe 

 that even a microscopic cell shows the tracings of the same 

 divine handiwork which swings the stars in their courses. 



Sclerogen Ligneous Tissue. Sometimes the deposit 

 within the cell-wall is of considerable thickness, and often 

 in concentric rings, through which a series of passages is 

 left so that the outer membrane is the only obstacle to 

 the access of pabulum, as in the stones of fruit, gritty tis- 

 sue of the pear, etc. (Plate VII, Fig. 96). The nature of 

 this deposit is similar to cellulose, although often contain- 



