CHAPTER XV 



the flower in place despite the struggles of entrapped guests. I have 

 always found it easy to capture bees on the flowers, and have never 

 failed to find traces of pollen masses on their legs; very often only the 

 little discs remain, the pollinia having been severed. 



The cactus is a type of plants that, without dying down to the 

 ground, can adapt themselves to a long dry season. Mexico and the 

 arid regions of America generally, have the greater part of all the 

 known species of cactus, but South Africa has many similar plants 

 belonging to the genera Ficoidese, Crassulaceae, Portulacaceae and the 

 like. Plants of this type are found also on the faces of rocks where 

 there is very little soil, and on sea sands where evaporation is rapid. 

 In order to present the least possible evaporating surface, in all these 

 plants cylindrical or spherical forms are approached. In the cacti it 

 is the stems that assume these forms, but in century plants, the Cras- 

 sulaceae (live-for-ever, hen and chickens, and the like), ice plants, 

 &c., it is the leaves. These plants must always have an epidermis 

 that restricts evaporation, either a very thick one, as in the cactus, 

 or one with a waxen or silicious coating. They have also what is 

 termed aqueous tissue, i. e., tissue that stores water. In the cactus, 

 this tissue is in the interior, its cells and their contents being trans- 

 parent; in many of the plants, the aqueous tissue is visible beneath 

 the epidermis as transparent lines or dots. The ice plant, Mesembry- 

 anthemum, common on our beaches and also cultivated in gardens, 

 has water stored in crystalline vesicles that thickly cover stem and 

 leaves; it is not fully understood why in this last case the water in 

 the delicate- walled vesicles does not evaporate easily; it is probably 

 because of dissolved salts in the cells and the nature of the cell walls. 

 Where these plants are exposed to the attacks of animals, silicious 

 coats or dissolved mineral salts sometimes serve defensive purposes, 

 but the cactus, like the yucca and century plant, has savage weapons. 

 Our common tuna cactus, Opuntia Lindheimeri, var. occidentalis, 

 has not only the long spines, which are modified leaves, but also at 

 the bases of the spines numbers of maddening, little, barbed bristles; 

 one of these bristles under the microscope is represented in Fig. 16. 

 It is not unusual for one cactus to have three kinds of weapons. 



For a few weeks in spring time, the tuna cactus has true leaves, 

 like little fleshy horns, but most cacti have only the spines. Frag- 

 ments of the tuna take root very readily ; they are easily detached 

 from the parent plant, and the spines must aid in their distribution. 

 Probably there are few seedlings. The flowers of the tuna open but 

 once, and then for a few hours only, but they are thronged with bees 

 that jostle one another in their eagerness for pollen; the bees usually, 



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