CYTOLOGY 



23 



spirally around the inside of the transparent cell wall, the threads 

 appearing under the microscope to cross and recross in beautiful 

 patterns. In reality, however, the threads do not touch each other. 

 After the cool October weather comes, one is likely to find in the 

 spirogyra a most interesting change in progress. During the spring 

 and early summer the spirogyra grows by a reproduction of its cells 

 by fission; these cells remain together and the reproduction thus 

 produces the long, hair-like threads. As these delicate threads cannot 

 live over winter, nature prepares for this season by causing a new 

 method of reproduction. Two cells lying side by side put out pro- 

 jections which fuse or join together, making a communication from 

 one cell into the other. Through this communication the contents 

 of one cell flows into the other and the contents of both cells are thus 

 mixed. This process is called " conjugation. " After the conjugation 



Diatoms. Having silicious envelopes or shells outside the exochrome. 



the new mass forms a thick cellulose covering and passes into a 

 "resting stage" for the winter. With the warmth of returning 

 spring the cellulose shell is burst and the new plant starts its 

 summer growth. 



Motion is so characteristic of the higher animals and a lack of 

 voluntary motion so characteristic of higher plants, one naturally 

 associates motion with animal life. But many of the lower orders 

 of plants have the power of locomotion, while many of the lower 

 animals e. g., corals and barnacles are as fixed as a tree. 



Among the algae (motile) are diatoms and oscillaria. 



Diatoms. These are one-celled plants possessing a brownish- 

 green chlorophyll and encased in two delicate silicious scales, which 

 fit together like a box and its cover. They are boat-shaped and 

 move slowly across the field of the microscope, pushed along by a 



