GROWTH AND WORK OF PLANTS 



end of the underground stem is brought to the surface of the 

 ground it will produce a normal green leafy shoot. 



84. Cacti. The cacti embrace a great variety in the form of 

 the stern. The stems are greatly specialized. They are succu- 

 lent (fleshy and contain quantities of water) stout and thick, 

 with a thick waxy cuticle on the surface. The stomates (par. 

 144) are deeply sunk in depressions. These characters aid the 

 cacti in the conservation of water which is of great importance 

 to them since they usually grow in desert or dry regions. 

 They are further specialized in that they do not have green 

 leaves, the function of the green leaves being performed by the 



stem which contains the chlorophyll 

 (par. 147). In some species the 

 prickles or spines which are so 

 numerous on the cacti are sup- 

 posed to represent leaves since they 

 are outgrowths of the stem. Some 

 of the cacti have tall stout columnar 

 stems, some are shaped like a melon 

 as the "melon cactus," others have 

 branched stems with flattened pear- 

 shaped joints as in the prickly pear 

 cactus. This is widely distributed 

 in dry regions of the West and 

 South. Examples of the cacti are 

 usually to be found in green- 

 houses. Such stems are some- 

 times called condensed stems. 



85. Other succulent stems. 

 Some succulent stems are common 

 in regions which are not habitually 



dry. The purslane is a common weed in the northern and 

 eastern United States. It has thick, smooth, watery stems, and 

 thick, small, succulent leaves. It is very difficult to kill because 

 of its power to conserve water. The houseleek, liveforever, 

 stonecrops, etc., are other examples. 



Fig. 5o. 

 Phylloclades of smilax. 



