THALLOPHYTES 11 



of the conditions necessary for its activity. Anything that 

 withdraws water from the cell diminishes its activity, and 

 if the loss of water continues, the protoplast becomes in- 

 active and may pass into the condition called dormancy. 

 In seeds, for example, the protoplasts may remain in this 

 dormant condition for a long time, and then become active 

 again when water is restored. 



11. Work of the cell. The work of the cell has been 

 referred to in describing its parts, but it is important to 

 emphasize it. The work of a plant of many cells is simply 

 the sum of the work of all its cells. The work done by 

 a living cell is so complex that it may be analyzed under 

 several heads, but all of it may be grouped under two heads. 



One is the work of nutrition, which includes everything 

 that has to do with the securing and using of food. It is 

 by this work that a plant maintains itself in vigor and in 

 growth. The principal part of the body of a complex plant 

 is concerned with the work of nutrition, and this part is 

 called the nutritive or vegetative body of the plant. 



The other kind of work is reproduction, which includes 

 everything that has to do with producing new plants. 

 In most plants the reproductive structures form a relatively 

 small part of the body, but in one-celled plants or in simple 

 many-celled plants nutrition and reproduction are carried 

 on by all the cells. 



12. The vegetative body of Algae. Algae may be ar- 

 ranged in a series, beginning with those having the most 

 simple bodies, and advancing to those having the most 

 complex bodies. In this way one can appreciate the prog- 

 ress made by the plant body, and also the amount of such 

 progress made by the Algae. For the sake of clearness, 

 we may think of the bodies of Algae as representing different 

 stages of general progress. 



The first stage is represented by those Algae whose bodies 

 are single cells. Such plants are represented in the illustra- 



