OPCNTIA. 83 



O. Tuna, Miller (Cactus Tuna, Linnasus). — This is a very strong-growing 

 species, and is one of the principal of those employed for fences and hedges, 

 as already mentioned. It is also one of the Cochineal plants, and in some 

 districts is more largely grown for that purpose than the Nopalea, par- ■ 

 ticularly in Mexico, its native country, but in many other parts of America 

 it is nearly as abundant. The branches are long, flat, and elliptical in form, 

 •with numerous yellowish spines. The flowers are large, pale yellow, or with 

 a rosy tint, and are followed by the characteristic " Prickly Pears " of the 

 genus. One interesting peculiarity of the stamens in this species was long 

 ago observed by Dr. J. B. Smith, then President of the Linneau Society, and 

 published by him in a volume of " Tracts Belating to Natural History " in 

 1798. In one of these the «uthor discusses the various phenomena connected 

 with the " Irritability of Vegetables," especially referring to the stamens of 

 Berberis communis. In connection with this he mentions, " The Barberry 

 is not the only plant which exhibits this phenomenon. The stamina of Cactus 

 Tuna, a kind of Indian Fig, are likewise very irritable. These stamina are 

 long and slender, standing in great numbers round the inside of the flower. 

 If a quill or feather be drawn through them, they begin, in the space of two 

 or three seconds, to lie down gently on one side, and in a short time they are 

 all recumbent at the bottom of the flower." I have tested this, and find that 

 the rapidity of the response to the stimulus varies greatly with the condition 

 of the weather at the time, and the age of the flower. In some cases the 

 stamens appeared to have quite lost the property ascribed to them, and in 

 others several minutes elapsed before there was any perceptible motion. 

 The peculiarity is, however, not confined to O, Tuna, some other species 

 exhibiting a similar sensitiveness, but the case is not quite analogous to that 

 of the Berberis, the arrangement in that Sower being more of a mechanical 

 nature. 



O. Salmiana, Parmentier. — One of the most attractive of the slender, 

 growing species with cylindrical stems is that upon which has been bestowed 

 the' name of a celebrated patron of the Cactus family. Prince Salm-Dyck. 

 It is a native of Brazil, and was introduced about 1850, having been received 

 at Kew from the continent about that time, and it has since then become 

 one of the most favourite forms of the group in cultivation. As shown in 

 fig. 15, which is a very slightly reduced representation of a portion of the 

 plant, the stems are very slender, and cylindrical, with numerous little tufts 

 of small spines, and bearing near the summit dense clusters of very pretty 

 neat open flowers. These are 1§ to 2 inches in diameter, creamy white, the 

 buds and outer surface of the petals being tinted with red, which gives a 

 most pleasing appearance to the plant. It requires to be grown in a warm 

 position, and needs a light soil, loam and leaf soil in equal parts having 

 been found to suit it admirably. It is well worthy of more extended 

 cultivation. 



O. AEBoaESCBNS (the 'Walking Stick or Blk-Hom Cactus) .—This species, 

 which grows in Colorado and contiguous districts, has a narrow much-branched 



