128 



Bulletin 79 



peratures of — 17'' C. (1.4° F.). No direct observations, however^ 

 have been made in the field at this low temperature. 



5. O.fusicaulis has a cuticle 10 to 12 (jl thick, a crystal-bearing 

 layer of 20 to 30 tx, and a thick-celled layer 87 to 98[x.in thickness 

 This species is not hardy. It is injured at a temperature of — 6° C. 

 (21° F.). 



6. O. ficus indica, from Sicily, has a cuticle 12 to 15 \i. thick, 

 a crystal-bearing layer 30 to 40 [x, and a thick-celled layer 

 62 to 74tx.in depth This species is not hardy; it is injured at tem- 

 peratures below —5° C. (23° F.). 



It will be observed that the cuticle and the thick-celled layer 

 show great differences in thickness, while the crystal-bearing layer 

 is more nearly of the same thickness throughout. 



The table below gives a summary of the thickness of the various 

 layers noted above. ^ 



SUMMARY OF THICKNESS OF CELL LAYERS IN CACTi 



Name of plant 



Opuntia castillae 



O. sp. Burbank Special. . . . 

 O. ficus indica, from Malta 



O. Ellisiana 



0. fusicaulis 



O. ficus indica, from Sicily. 



Thickness 

 of cuticle 



Thickness 

 of crystal- 

 bearing layer 



15-20 (A 



5- 8 iJL 



5-10 IX 



12-15 (X 



10-12 (x 



12-15 tx 



30-42 (X 

 30-38 [X 

 30-38 [X 

 25-35 IX 

 20-30 (X 

 30-40 (X 



Thickness 



of thick-celled 



layer 



100-120 lA 

 70- 87 n 

 55- 75 n 

 75-130 I* 

 87- 98 {1 

 62- 74 (fc 



RELATION OF PENETRATION OF TEMPERATURE TO THICKNESS OF 



INTEGUMENT OF CACTi 



To determine whether the length of time of penetration of tem- 

 perature is influenced by the thickness of the integument or not 

 the following experiment was performed. Pieces of the integu- 

 ment about 5 by 5 cm. in extent were dissected from the joints 

 of all of the above Opuntias. The integument of each species was 

 wrapped carefully around the bulb of a thermometer and tied 

 with a thin wire. Such prepared thermometers were put quickly 

 in large test tubes the openings of which were closed with plugs 

 of cotton. The test tubes had been in an ice box at a temperature 

 of 4° C. (39.2° F.) several hours before the thermometers 

 were placed in them. Aft^r the thermometers were inserted 

 in the test tubes and the openings closed with cotton, the 



