220 



THE TEACHING BOTANIST 



f raw materials - - ducts. 

 Conducting : \ 



( food materials sieve tubes. 



Aeration - - intercellular passages and stomata. 



Excretion - - special crystal cells. 



Storage- -pith, medullary rays (and cortex in roots). 



Experiment No. 7.- -Prepare a plant as shown in Fig. 21, 

 i.e. place it in a glass jar and cover with dentist's thin sheet 



rubber, tightly tied both to 

 jar and to plant, but pierced 

 by a thistle tube which is 

 closed by a piece of the rub- 

 ber (or cork) that can be 

 removed. All water must 

 then come out through the 

 leaves and stem. Place 

 the plant on the scale pan 

 of a good balance (the 

 Harvard trip-scale, used in 

 elementary courses in phys- 

 ics, is good), and weigh 

 it at intervals. Add water 

 through the tube, carefully 

 weighing the plant before 

 and after, to find the 

 amount added ; it is very 

 essential to the health of 

 the plant not to add too 



FlG. 21. Method of preparing a plant for i -, ,, . , 



much, and this can be 



transpiration experiments by weighing. XJ. 



judged either by the 



amount given off, or by the appearance of the earth in the 

 pot, which should be allowed to become nearly dry between 



