20 THE PLANT CELL 



example of this kind. When certain kinds of bacteria are mounted 

 in a drop of water under a coverglass they are found to assemble 

 about the edge of the coverglass to get oxygen; in which case the 

 stimulus is of a chemical nature; and in fertilization the find- 

 ing of the egg by the sperm cell seems to be due to the perception 

 of a chemical substance excreted by the egg. The growth of 

 roots toward the center of the earth and of shoots away from it 

 is due to a perception of the direction of the gravity pull. Many 

 other instances could be given for stimuli of different natures; 

 but the few here cited will do to show the fact of perception and 

 response by the protoplasts. This power of the protoplasts is 

 called irritability. 



(g) Growth. — When protoplasts are permanently increasing 

 in size and assuming their permanent forms they are growing. 

 Examples of this are seen where cells at a growing, apex or 

 in the cambium region are increasing in size and taking on the 

 forms characteristic of the different tissues which they are 

 to compose. * Of course the size and form of the whole plant 

 body are due to the growth activity of its individual protoplasts. 



{h) Reprodiiction. — We have already seen that the protoplast 

 is able to reproduce its kind by self-division, where the behavior 

 of the nucleus indicates that the process is one of considerable 

 complexity. 



Illustrative Studies 



1. Cut free-hand sections (see page 251) of bottle cork from 

 the three points of view illustrated in Fig. 123, and mount them 

 for study in a drop of dilute glycerine (half glycerine and half 

 water). Draw three or four cells from each point of view. If 

 the microscope is equipped with a micrometer eyepiece — and 

 it should be for the wOrk of this book — draw to scale as explained 

 on page 278. In drawing cell groups outline the cell cavities, 

 leaving a space between adjoining cells to represent the thickness 

 of the cells, as shown in Fig. i, B. 



2. Study longitudinal sections of onion root tip prepared 

 as directed under Cytological Methods on page 257. Or if 



