442 



HOUSE-LEEK TRIBE ; ORDER CACTE.E. 



exist in Britain, belong to the Linnaean class and order DECAN- 

 DRIA Pentagynia, having five parts in the calyx, corolla, and 

 ovarium, with double the number of stamens ; whilst the Sem- 

 pervwum, or House-leek, is found under DODECANDRIA Dode- 

 cagynia, having twelve stamens and twelve styles. The common 

 British species of the latter is remarkable, for almost always 

 bearing ovules on its anthers, instead of pollen. These plants 

 are not only very tenacious of life when naturally exposed to the 

 elements, but resist efforts made to destroy their vitality, when 

 it is desired to preserve them. The collector finds it very diffi- 

 cult, therefore, to prepare them for his Herbarium ; for they 

 wili push long shoots whilst under great pressure, and after 

 being submitted to a high temperature. It is from the defi- 

 ciency of stomata, and the thickness of the cuticle, that it is so 

 difficult to dry them ; and it is obviously by these properties, 

 that they are enabled to resist the rays of the sun. 



610. The order CACTE^E, or Prickly Pear tribe, is by nature 



exclusively confined to the 

 New World ; but several spe- 

 cies have now been naturalised 

 elsewhere. They do not, even 

 on that continent, extend far 

 from the tropics ; they fre- 

 quent hot, dry, exposed situa- 

 tions, like those to which the 

 Crassulacese are adapted. Most 

 of the species of this order are 

 remarkable for the absence of 

 leaves, of which no other traces 

 are found, than tufts of prickles 

 arising at regular intervals 

 from the stem, .these being 

 the veins of the leaves, be- 

 tween which the parenchyma 

 is not developed (. 236). 

 The stems and branches are very fleshy and succulent, and 

 usually have flat expanded surfaces, which in some degree per- 



Pre. 162. STEM, BRANCH, AND FLOWSR OF 

 CACTUS. 



