HOW PLANTS GROW. 219 



The protoplasm, utricle, and nucleus are the three contents 

 which play the important part in the origin and growth of all new 

 cells. 



5. — The Vacuoles. When the protoplasm has reached its 

 limit of absorbing power for water, it exudes this water in the 

 form of drops of varying size in its interior. These form the 

 vacuoles, which are cavities in the protoplasm, and contain — 



6. — The Watery Cell-Sap. This may be in many small 

 vacuoles, or in one large one, filling the cavity bounded by pro- 

 toplasm, and formed from it by exudation. The sap — the 

 " crude " sap, as it is called — is the fluid needed for the life of the 

 plant ; it is absorbed by the roots, and carried up by the stem. 

 In it are suspended all the other contents — those taken in with it 

 by the roots, and those formed inside the cells themselves. There- 

 fore, it is never found unmixed. When it has reached the leaves, 

 it is, under given conditions, transformed into " elaborated " or 

 unmixed sap, containing the materials of growth ready for use. 



We thus see that a typical plant-cell is fitly described as con- 

 sisting of — 



^.^The Cell- Wall, lined by 



2. — The Primordial Utricle, which is the outer layer of 



3. — The Protoplasm, at the centre of which is 



4. — The Nucleus, with usually a nucleolus ; 



5. — The Vacuoles, cavities in the protoplasm, and containing 



6. — The Watery Cell-Sap, a nutritive fluid. 



The Cell-Wall is simply the framework of the minute work- 

 shops in which the marvellous operations of plant-life are carried 

 on. 



Size. — Cells vary in size from i-6,oooth of an inch in dia- 

 meter (as seen in Fungi spores) to i-3oth of an inch, as seen in 

 some gourds and aquatic plants. Elder pith shows them i-iooth 

 of an inch, and ordinary cellular tissue i-4ooth of an inch. In 

 length, they range from i-5oth of an inch long, as in wood-cells, 

 to i-i2th or even 2-3rds of an inch, as in the liber-cells of 

 flax. Some unicellular water-weeds, being many-branched, may 

 measure some inches in length, while their diameter may be 

 expressed in hundredths of an inch. Taking i-i 200th of an 

 inch as a small diameter for ordinary tissue ; one cubic inch of 

 such tissue would contain 1728,000,000 cells. 



