STEUCTUKE. 3 



Cereus. The surface is either marked with angular ridges from base to 

 summit, upon which are arranged with great regularity a series of clusters 

 of spines varying in size, colour, and number, or, as in the Mamillariag, 

 the surface is broken into a number of small rounded projections or 

 mamilliB, each crowned with a, cluster of spines. These spines in 

 several of the genera furnish useful characters in distinguishing the 

 species, size, colour, and number, being found to be constant in the 

 majority of cases. They are exceedingly numerous, and specimens of 

 moderate size of Cereus senilis have been found to have from 50,000 to 

 70,000. Iq size, too, the Cactese have a wide range of variation, from some 

 of the diminutive Mamillarias a few inches high to the gigantic foms of 

 Cereus which are found in their native habitats upwards of 50 feet high, 

 and the huge Echinocactus Visnaga, single specimens of which have been 

 introduced to this country weighing as much as a ton. These plants 

 contain comparatively little woody tissue except when they are very much 

 advanced in age, the cellular tissue being very largely developed in the 

 majority of the typical forms, the cells beicg large and mostly filled with 

 a clear water-like fluid, but in others with milky mucilaginous or 

 slightly acrid juices. There is also usually a quantity of crystals of oxalate 

 of lime, which are readily seen if a stem is cut after being dried. Having 

 no leaves the function of respiration must be performed by the stem ; but 

 as this has to serve as a reservoir of nutriment generally exposed to a 

 burning tropical heat, it is obviously of importance that the evaporation 

 from the surface should be reduced to a minimum. As a means to this 

 end we find that the stomata or breathing pores, which are so abundant 

 on the leaves of most plants inhabiting temperate climates, are in the 

 CactesE comparatively few. To this, together with a peculiar structure of 

 the walls of the cells forming the superficial layer, is due the remarkable 

 heat and drought-enduring character that enables them to live and thrive 

 where most other vegetation would perish. 



The flowers differ much in size and colour, but they mostly agree 

 ia the large number of sepals, petals, and stamens, both the former 

 very frequently coloured alike, and only distinguishable with great 

 difEioulty. The colours most abundant are shades of rose, crimson, 

 purple, and yellow, some being white and others greenish. Many flowers 

 possess a powerful and most pleasing fragrance, but they are usually 

 of extremely short duration, some lasting but a few hours during the 

 evening or night. The fruit is of a fleshy substance, and in the case of 

 several species, such as Cereus speciosissimus and Opuntia vulgaris, it is 

 edible, and when well ripened of u very agreeable flavour, somewhat 

 resembling that possessed by some fruits of the Gooseberry family, to 

 which the Caoteas are distantly related. 



