STANDLEY — TREES AND SHEUBS OP MEXICO. 855 



3. Petalonyx linearis Greene, Bull. Calif. Acad. 4: 188. 1885. 



Baja California and adjacent islands. 



Low shrub, very scabrous throughout; leaves 1 to 3 cm. long, obtuse, 

 very thick; racemes sometimes 12 cm. long, the bracts rounded-cordate, 

 entire ; petals yellowish white, 4 to 5 mm. long. 



Some of the Mexican species of Begonia are perhaps to be classed as 

 shrubs, but there is no satisfactory evidence to this effect. 



116. CACTACEAE. Cactus Family. 

 (Contributed by Dr. N. L. Britten aiid Dr. J. N. Rose.) 



References: Britton & Rose, The Cactaceae, vols. 1-1. 1919-23; Safford, 

 Cactaceae of northeastern and central Mexico, Ann. Rep. Smiths. Inst. 1908: 

 525-563. pJ. 1-15. 1909. 



Plants perennial, succulent, usually shrublike or treelike; leaves usually 

 none or much reduced, sometimes large and flat ; spines present, variable in 

 form and arrangement, borne upon areoles in the leaf axils; flowers usually 

 perfect, regular or irregular, solitary or clustered, sometimes borne in a 

 terminal specialized inflorescence known as cephalium ; perianth tube none 

 or large and elongate, with few or numerous lobes, these sometimes differ- 

 entiatetl into sepals and petals; stamens commonly numerous, the filaments 

 usually borne upon the throat of the perianth, the anthers small, 2-celled; 

 style one, terminal, the stigma with 2 to many lobes; ovary 1-celled ; fruit 

 baccate, fleshy or dry, usually containing numerous seeds. 



The Cactaceae constitute one of the largest and most interesting gi-oups 

 of Mexican plants, and they are more profusely developed in Mexico than 

 in any other part of the earth. They are often conspicious features of the 

 landscape, particularly in the desert regions, and they are of great economic 

 importance. Many of the more showy ones are commonly grown in gardens 

 i^or ornament. 



Leaves broad, flat; flowers stalked (stalk sometimes very short). Areoles 

 without glochids. (Pebeskieae) 1. PERESKIA. 



Leaves (except in Pereskiopsis) terete or subterete, often wanting; flowers 

 sessile. 

 Areoles with glochids; vegetative parts bearing leaves, these usually small 

 and fugacious; flowers without a definite tube. (Opuntieae.) 



Leaves broad and flat 2. PERESKIOPSIS. 



Leaves subulate or cylindric. 



Stamens much longer than the petals 3. NOPALEA. 



Stamens shorter than the petals. 



Joints of the stems flat to terete, not ribbed 4. OPUNTIA. 



Joints terete, longitudinally ribbed 5. GRUSONIA. 



Areoles without glochids; vegetative parts usually without leaves; flowers 

 with a deflnite tube (except in Rhipsalis). (Cereeae.) 

 Perianth rotate or nearly so, without a tube. Plants slender, spineless. 



epiphytic, many-jointed 54. RHIPSAXIS. 



Perianth not rotate, with a deflnite tube. 

 Areoles mostly spineless ; plants with numerous joints, these flat ; epi- 

 phytes. 

 Tube of flower deflnltely longer than the limb___51. EPIPHYLLUM. 

 Tube of flower not longer than the limb. 



Perianth campanulate, with few segments 52. CHIAPASIA. 



Perianth short-funnelform, with many segments. 



53. NOPALXOCHIA. 



