2 2 Elementary Plant Physiology. 



part of this section. A sliorter, 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 steins. — The most casual 

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

 stems are us ually thicker at the base than abov e. This is due 

 to the fact that increase in thickness takes place during every 

 vegetative season, and the oldestjiortionsJiasE-natuially accom- 

 plShe3-tIie'^greatest_Jii€rease— in bulk. Such increase in the 

 wood of a tree-trunk takes the form of the " an nual cy hnders, 

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

 tion of bark. 



Examine the younger twigs of any convenient tree. The 

 surfaces will be sgeo^t p appear_ _smooth, and are gr een in man y 

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

 and note that the outer portion of the ste m is r nade_j]g_of 

 ljving_ce]ls, in which the protoplasm is stained yellowigh brown 

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

 Underneath the c ortical and o uter region of the st em is the^ 

 t]ain__sheet of dejicate^jcambium surrounding the central C3-1- 

 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 conjisto f^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 



