74 APPLIED BIOLOGY 



73. Microscopic Structure of Bean Leaf. (D) With fine-pointed 

 forceps strip off a very small piece of the thin transparent epidermis 

 from both the upper and lower surfaces of a leaf, and mount for 

 microscopic study. This will present surface views of the leaf. 

 With low power, note the irregular cells of which the epidermis is 

 composed (compare with Fig. 29, which is from other leaves). 



Scattered among these irregular cells are pairs of crescent-shaped 

 cells set together so as to leave a small pore or opening through the 

 epidermis. Each pore is a leaf-pore or stoma (a Greek word mean- 

 ing an opening, plural stomata). It opens into a small cavity (air- 

 space) beneath the epider- 

 mis. Are the leaf-pores 

 present and equally abun- 

 dant on both upper and 

 lower sides of the bean- 

 leaf? 



These two crescent- 

 shaped cells around the 

 pore are called guard-cells. 

 It has been noticed that 

 FIG. 29. Epidermis from certain plant leaves. u watpr ;<, ohnnHanf in 

 A, upper surface. B, lower surface with when water is abundant m 

 stomata. (From Strasburger.) th e plant these cells swell 



and become more crescen- 



tic in form, leaving a larger opening than when the leaf is drier. It 

 is believed by most botanists that the guard-cells are able to prevent 

 excessive evaporation of water from the air-spaces, and thus conserve 

 water when necessary. 



Make a drawing of a group of cells from the epidermis, including 

 at least one pair of guard-cells. 



Cut off a leaf transversely, and with a strong hand-lens examine 

 the cut end. Note the transparent epidermis which covers both 

 sides and the edges ; that is, entirely surrounds the leaf. Between 

 the upper and lower epidermis the center of the leaf appears to be 

 filled with a green-colored, somewhat granular material (middle- 

 tissue or mesophyll). Also notice the cut ends of the colorless veins. 

 If the hand-lens is a strong magnifier, it is possible to see 

 that the green-colored middle-tissue is more compact toward the 

 upper surface and appears to have small cavities in the part next 

 the lower epidermis. This will be very clearly seen when the com- 

 pound miscroscope is used for examining the cut end of a leaf. 



(Z)) Examine with a miscroscope a very thin transverse section 

 from the end of a leaf cut like the one described above. [To cut 



