36 



THE AMERICAN NATURALIST 



[Vol. XL 



ticularly on account of the small rate of transpiration which 

 was demonstrated in this plant. Echinocactus has a heavy outer 

 membrane which is cuticularized 

 (Figs. 9, 10). The stomata are 

 peculiarly fitted to guard well a 

 too rapid loss of water, and they 

 have a somewhat complicated 

 structure. The stoma as charac- 

 teristic of many xerophy tic plants, 

 is sunken below the general sur- 

 face of the stem, and from it in 

 Echinocactus a tube, divisible in- 

 to two portions of separate func- 



Tiew^stoma!'^"'^'"* w'is'j^eni. Surface ^.^^^ j^^^^ deeply into the OUter 



portion of the cortex and becomes 

 the peripheral portion of the extensive intercellular aerating sys- 

 tem of the plant. The substomal tube, really trachea, is shown 

 schematically in Fig. 11. The outer portion, which is heavily 

 shaded in the sketch, is cuticu- 

 larized throughout its course in 

 the sclerenchymatous tissue (Fig. 

 10) of the cortex, and in this part 

 it acts merely as a tube for the 

 conduction of gases. Beneath 

 the supporting tissue the tube 

 enters the chlorenchyma and its 

 wall is no longer cuticularized; it 

 here functions as the substomal 

 chamber proper. 



It is of interest to contrast 

 with this permanent structure a ^jg 9 — Echinon„fn^- in^i;-,,,, 



form of stoma and sort of adjoin- mataandsut.stonMl , amil uln, l, 



ing tissue which are a part of Fiu!7H.ni'^n"!!n'''i!l'i'i'v\,ia^^^^^^^ 

 evanescent organs, and which also I'yma . i is,-, .s , 



apj)ear to have somewhat (litVen-nt fuiu tioiis. I liuve reference 

 to the stomata of the leaves of OpuNtlu rrrsicolor. The perma- 

 nent stomata of this Opuntia closely resemble those of Echino- 

 cactus but the temporary stomata, those of the leaves, are verv 



