THE CELL. 161 



examining plant-fiairs and other parts under high powers 

 of the microscope, when it will be seen that there are 

 movements of two kinds. The whole mass of protoplasm 

 has a rotary motion, sliding upon the cell- wall, down- 

 wards on one side and upwards on the other. This is 

 the mass-movement. Also, currents may be traced passing 

 across the protoplasm in different directions. This is 

 the streaming-movement. 



In some of the very lowest plants, where there is no 

 cell-wall, and the whole is a mass of naked protoplasm, 

 these movements may be observed more readily because 

 they are less restricted. 



263. There is some doubt as to the exact chemical 

 composition of protoplasm. It is, however, a very complex 

 substance belonging to a group of bodies known as 

 albuminoids, of which nitrogen is an important con- 

 stituent. 



The consistence of protoplasm depends upon 

 the amount of water it contains. In dry seeds, 

 for example, it is tough and hard, but when 

 the same seeds are soaked in water it becomes 

 partially liquid. 



264. Forms of Cells. As cells become 

 older they tend as a rule to change their form, 

 though sometimes we find them differing but 

 little from their original conformation. Com- 

 monly a cell grows more rapidly in some one 

 direction, thus giving rise to long forms, as is ^'S- 221. 

 the case in stems generally, and in the petioles and veins 

 oi leaves, the superior toughness and strength of which 



Fig. 221.— Prosonohyma ot the wood. (Gray.) 



