136 INTAKE AND DISTRIBUTION OF GASES 



the low power objective, determine the number of stomata in 

 one sq. mm. Measure the stomatal apparatus and the opening. 

 Draw a stoma together with the surrounding epidermal cells, 

 using the high power. Turn the leaf upper side up and deter- 

 mine the number of stomata as before. 



2. Study the stomata in leaf cross sections. Sections cut 

 free-hand will do. Mount several of the thinnest sections in 

 glycerine and with the high power find a median section through 

 a stoma. What provision do you find for the distribution of 

 gases after they have entered through the stoma? Compare 

 the stoma with the diagram of Fig. 63, ^. Would it probably 

 work as there set forth? Draw the stoma and epidermal cells 

 adjoining it and the intercellular spaces leading from it. 



3. Look for intercellular spaces in the permanent prepara- 

 tions prepared for the previous chapters and make drawings 

 and measurements of some of them. Do you find every cell 

 somewhere touching an intercellular space ? 



4. Study intercellular spaces in the stems of water lily, Juncus, 

 or other water and marsh plants. Permanent preparations 

 or free-hand sections mounted in dilute glycerine may be used. 

 Draw some of the spaces by outlining the cells bordering them. 

 Estimate the percentage of stem volume occupied by these spaces. 



To show that the intercellular spaces in the leaf communi- 

 cate with those in the stem fill a bottle with water, push in a 

 perforated cork so that its upper surface is below the mouth 

 level; cut off the shoot of a young Ricinus plant and insert the 

 stem through the hole in the cork; remove all water from the 

 top of the cork and pour over it melted rosin one part and bees- 

 wax one part, so that no air can enter the bottle except through 

 the intercellular spaces of the plant; set the plant in the sun 

 but shade the bottle. As water evaporates through the leaves 

 air can be seen bubbling from the submerged end of the stem. 

 Explain how this happens. 



5. Examine lenticels on any woody stem that shows them 

 well, — elderberry, for instance. Study with a high power a 

 thin cross section of a lenticel and draw it. 



