CACTACEAE 103 



fleshy, of various shapes, rarely bearing green 1. , and usu. provided 

 with sharp barbed thorns, which give protection against animals. 

 We may consider briefly some of the more important types of shoot 

 found in C. (refer to genera for further details). The nearest approach 

 to the ordinary plant-type is perhaps Pereskia, which has large green 

 1., somewhat fleshy, in whose axils are groups of thorns mixed with 

 hairs ; the space occupied by these is termed the areole. About the 

 morphology of the spines there has been much dispute; most authors 

 regard them as repres. the 1. of the axillary shoot, whose stem is 

 undeveloped, but there is also good evidence in favour of the view 

 that they are "emergences." In some gen. they are provided with 

 barbs. The next stage is found in Opuntia, where the stem has 

 taken over the water-storing and assim. functions, but still bears 1. ; 

 in some sp. these aid the stem functions throughout life, but in most 

 they fall off very early, and the stem is usually flattened to expose 

 more surface to air and light. Then we come to Leuchtenbergia, 

 which has an aloe-like habit with the areoles on the tips of the 

 apparent 1. ; the fl. arises either in the axil of the "leaf" or on the 

 areole. Development shows that the apparent 1. is really a cpd. 

 structure. The bud stands, not exactly in the axil, but on the base 

 of the 1., and the two grow out together to form a leaf-cushion or 

 mammilla^ at the outer end of which is the growing point and the 

 rest of the 1. itself; the latter is represented by a small scale (often 

 microscopic) and the former gives rise to the thorns, &c. on the areole. 

 The same phenomenon is seen in Mammillaria, Cereus sp., &c. In 

 some cases the growing point divides, during the growth of the 

 mammilla, into two, one on the tip, the other in the axil, of the 

 cushion. The latter gives rise to the fl. In Cereus, Echinocactus, 

 &c. the stem is cylindrical, bearing ribs on which are the areoles at 

 regular intervals; the rib is formed by the "fusion" of mammillae, 

 i.e. by the growth of the tissue under them during their development 

 (cf. formation of sympetalous corolla). In Phyllocactus, Epiphyllum, 

 and sp. of Rhipsalis some or all of the shoots exhibit a flattened leafy 

 form with areoles in notches on their edges. This form appears to be 

 derived from the preceding by abortion of some of the ridges, and 

 reversions are often seen (they appear if access ot light lie prevented). 

 Lastly, other sp. of Rhipsalis show perfecily cylindrical stems. 



The bulk of the internal tissue consists of parenchyma in which 

 water is stored ; the cell-sap is commonly mucilaginous, thus further 

 obstructing evaporation. The cuticle is thick, and the ridges of the 

 stem are usu. occupied by mechanical tissue, whilst the stomata are 

 in the furrows. Everything thus goes to check transpiration to the 

 utmost extent ; it is very difficult to dry a cactus for the herbarium, 

 and its vitality is very great. Its growth is slow, but sp. of Cereus, &c. 

 reach a great size. Veg. repr. is frequent in the mammillate forms, 

 and occurs to some extent in others. In garden practice, cacti are 

 often multiplied by cuttings, for a piece cut off and stuck into the 

 soil will usually grow. Grafting is also largely resorted to. 



Fls. usu. solitary (exc. Pereskia), borne upon or near the areoles 

 or in the axils of mammillae, large, brightly coloured, ? , reg. or !. 

 P (oo ), showing gradual transition from sepaloid to petaloid 1., spirally 



