INTRODUCTION 



If 



8, Definitions. — "Organ" is a general name for any 

 part of a living thing, whether animal or vegetable, set 

 apart to do a certain work, as the heart for pumping blood, 

 the lungs for breathing, or the stem and leaves of a plant 

 for conveying and digesting the sap. By "function" is 

 meant the work or office that an organ has to perform. 



I. — Typical cells: k, nu- 

 cleus ; p, protoplasm ; w, 

 cell wall ; s, sap. 



9. The Cell. — In its strictly scientific sense this word is 

 applied to the smallest portions of organized matter that 

 go to make up a living body, whether vegetable or animal. 

 It usually consists of a tiny membra- 

 nous sac lined with a living semifluid 

 substance called protoplasm, which 

 ordinarily has one portion of denser 

 consistency than the rest, called the 

 nucleiLS. Within the protoplasmic 

 lining are contained various watery 

 fluids known as cell sap. These little 

 sacs are packed together to build up 

 the vegetable or animal structure as 

 bricks are in building a wall. They 

 are of various sizes and shapes. The 

 containing membrane is called the 

 cell wall. Cells can exist, however, without any wall, as 

 mere specks or globules of protoplasm, but these are not 

 common in vegetable structures. The essential part of 

 every cell is the protoplasm with its nucleus. This sub- 

 stance, so far as we know at present, constitutes the physi- 

 cal basis of all life, and if the protoplasm loses its vitality, 

 the cell dies and can no longer perform its functions of 

 absorbing and retaining liquids. Slice a fresh beet in a 

 vessel of water and a boiled one in another; how is the 

 liquid affected in each ? Account for the difference. 



The name "cell" is also applied to the compartments 

 into which the fruits and seed vessels of many plants are 

 divided. This double meaning of an important term is 

 unfortunate, but the context will always show in which 

 sense it is to be taken, so that no confusion need result. 



