STOMATA. 131 



the BIgelow's opuntia, so thickly In fact, that in making the 

 ascent, great care was necessary to escape sliding into them 

 by reason of the insecure footing. The sahuaro and a num- 

 ber of other plants descended into the huge well-like craters 

 occupying the floor at a level but a few feet above the sea. 

 Low stretches in the valleys were occupied by a rank growth 

 of the galleta grass, affording a coarse food for our horses. 

 Opuntia kiinzei, unknown and new at the time, was encoun- 

 tered in abundance, torote prieto and torote bianco, small 

 shrubs {Biirseras) occupied the hillsides, while Covillea 

 formed a mosaic over the whole region. 



Toward the Gulf was to be encountered a band of sand 

 dunes sixty and eighty feet in height with a coastal belt of 

 salt-loving and halophytic species beyond a broad zone of 

 galleta grass. 



A general view of the entire region shows that a number 

 of species extend over its entire breadth from elevations of 

 3000 and 4000 feet in southeastern Arizona to the dunes 

 near the shore of the Gulf, a distance of 200 miles. Among 

 these are to be included Cereiis giganteus, Covillea tridentata, 

 Encelia farinosa, Prosopis, Fouqiiieria, Parkinsonia, all of 

 which are essentially desert species, showing marked xero- 

 phytic structures. The highly localized distribution of the 

 greater number of forms, makes it extremely difficult to make 

 an analysis of their inter-relations and limits. The vegeta- 

 tion is essentially subtropical and a few of the forms men- 

 tioned extend far to the southward in this coastal desert. /\ 

 large number of annuals, active only during the rainy seasons, 

 are included. 



STOMATA AS REGULATORS OF 



TRANSPIRATION. 



By Francis E. Lloyd. 



The history of plant anatomy is not older than the 



knowledge that, leading from within the plant to the outside 

 are numerous openings whose appearance is now a matter 

 of elementary experience to every one more than superficially 



