610 



CACTUS 



CACTUS 



717. Opuntia leptocaulis, showing 

 sheathed spines. 



veins, as in Pereskia acu- 

 leata (Fig. 714), or sessile 

 and fleshy with only the 

 midrib and several paral- 

 lel nerves apparent as in 

 the genus Pereskiopsis. 

 They are sometimes 

 caducous, fleshy, cylindri- 

 cal or awl-shaped, as in 

 the genus Opuntia (Fig. 

 715). In the axils of the 

 leaves are peculiar cush- 

 ion-like areoles (corres- 

 ponding in all probability 

 to aborted branches) 

 clothed with down or felt- 

 like wool, from which 

 spines, and, in some gen- 

 era, also flowers, issue. In 

 the genera Opuntia and Pereskiopsis, the areoles also 

 bear minute short barbed bristles called glochidia, 

 which will penetrate the 

 skin and become detached 

 at the slightest contact and 

 are the source of annoying 

 irritation which often per- 

 sists for many hours! 



The spines (Fig. 716) 

 are not connected with 

 the axis of the stem or 

 branches, but emerge from 

 the areoles. In some 

 forms they are simple and 

 straight, bristle-like, awl- 

 shaped, or short and coni- 

 cal. In others they are 

 bent like fishhooks or are 

 curved and horn-like, with 

 transverse ribs. Some- 

 times they are minutely 

 downy or hairy and some- 

 times even plumose or 

 feathery. They may be 

 either naked or enveloped 

 in a membranous barbed 

 sheath (Fig. 717). They 

 may be grouped in star- 

 like clusters, with straight 

 or curved rays spreading 

 from a common center, or 

 in comb-like fascicles, with 

 the radial spines arranged 

 in two rows on each side 

 of a longitudinal axis (pec- 

 tinate) . In addition to the 



720. Leuchtenbergia principis, showing 

 transformation from scales to petals. 



radial spines, there are 



usually erect central spines 



either straight and rigid, 



or more or less curved. One of the most striking forms 



is that of the organ cactus, Myrtillocactus geometrizans, 



in which the stout erect 

 central spine resembles 

 the blade of a dagger 

 and the radials a guard 

 for the hilt. In contrast 

 with this may be men- 

 tioned the spines of 

 Pelecyphora aselliformis, 

 which resemble minia- 

 ture sow-bugs, or aselli 

 (Fig. 718). 



The flowers in most 

 cases issue from the 

 upper portion of the 

 areoles, but in certain 

 mamillarias and allied 



718. Extreme condensation of the plant body. 

 Pelecyphora aselliformis. (Nat. Size.) 



forms they come 



forth from between 



the tubercles or 



from their base at 



the end of a dorsal 



groove. Usually the 



flowers are solitary 



and sessile, but in 



the genus Pereskia 



(Fig. 714) they are 



ped uncled and often 



clustered. They 



may be tinted with 



rose-color, crimson, 



purple, yellow or 



orange, or rarely 



with copper-color or 



scarlet, but they are 



never blue. Often 



they are pure white 



at first, gradually becoming suffused with rose-color 

 in age. In a few species 

 they are inconspicuous, as 

 in the epiphytal Rhipsalis 

 (Fig. 712). Some are diur- 

 nal, others nocturnal; some 

 open at sunrise and close 

 at night or when the sky 

 becomes clouded; others 

 open at a certain hour and 

 close at another fixed hour 

 of the day or night; some 

 last for only a few hours, 

 others for a day, and 

 some persist for several 

 days. Some, like the 

 "night - blooming cereus" 

 are delightfully fragrant, 

 while others are ill-smell- 

 ing or have no perceptible 

 odor. 



The perianth is not 

 divided sharply into calyx 

 and corolla, although the 

 outer floral leaves are usu- 

 ally sepal-like and the 

 inner ones are true petals. 

 In one great division of 

 the family including Opun- 

 tia, which has been named 

 Rotatiflorse, the perianth 

 is more or less wheel- 

 shaped or widely spread- 

 ing (Fig. 719) ; in the other 

 division, Tubuliflorse, to 

 which Cereus belongs, the 

 floral leaves form a 

 tube, often 



remarkably long and slender, and crowned 



with a spreading limb. The floral leaves 



are not arranged in definite series but 



somewhat like those of a water-lily, the 



scale-like lower or outer leaves gradually 



becoming broad and petaloid as they 



approach the center (Fig 720). In all 



cases the perianth crowns the ovary, 



and sometimes persists after withering 



on the apex of the fruit (Fig. 721). The 



stamens are very numerous and are 



inserted on the petals or perianth-tube 



(Fig. 722). The single style is longer 



and stouter than the slender filaments, 



and usually terminates into a radially 



divided stigma (Fig, 723). Sometimes 721 



the stigma is conspicuously colored and Cephalocereus 



issues star-like from the center of the fruit. 



