22 Elementary Plant Physiology. 



part of this section. A shorter, stouter strip of wood should 

 be used, or a glass rod may be substituted for it. 



The reading of the mirror auxanometer may be taken in 

 another way if desired. To do this, remove the peepsight, and 

 set a lamp with reflector exactly in its place. Darken the 

 room, and note the interval covered by the reflection. 



The continuance of any of the above measurements during 

 more than one day involves the readjustment of the plant to 

 the instrument. This may be accomplished by setting the pot 

 on a base consisting of half a dozen small sheets of thin glass, 

 at the beginning of the experiment, and removing one or more 

 of them to throw the mirror reading back to zero. 



7. Increase in thickness of stems. The most casual 

 glance at any tree of the dicotyledonous type will show that the 

 stems are usually thicker at the base than above. This is due 

 to the fact that increase in thickness takes place during every 

 vegetative season, and the oldest portions have naturally accom- 

 plished the greatest increase in bulk. Such increase in the 

 wood of a tree-trunk takes the form of the " annual cylinders, 

 or rings," of wood, and is also partly accountable for the forma- 

 tion of bark. 



Examine the younger twigs of any convenient tree. The 

 surfaces will be seen to appear smooth, and are green in many 

 species. Cut a thin cross-section from a place near the apex, 

 and note that the outer portion of the stem is made up of 

 living cells, in which the protoplasm is stained yellowish brown 

 by a drop of iodine solution run in under the cover glass. 

 Underneath the cortical and outer region of the stem is the 

 thin sheet of delicate cambium surrounding the central cyl- 

 inder. Cut a thin cross-section from a place 2 or 3 cm. from 

 the tip, and examine as before. The outer portion of the 

 stem appears to consist of dead and dying cells, as may be seen 

 by tests for protoplasm with the iodine solution. Such tissues 

 are, of course, incapable of further expansion, and hence are 



