4 6 



GROWTH AND WORK OF PLANTS 



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

 with a thick waxy cuticle on the surface. The stomates are 

 deeply sunk in depressions. These characters aid the plant in 

 the conservation of water which is of great importance to these 

 plants since they usually grow in desert or dry regions. They 

 are further specialized in that they do not hi 'e green leaves, 

 the function of the green leaves being performed by the stem 

 which contains the chlorophyll. In some species the prickles or 

 spines which are so numerous on the cacti are supposed to rep- 

 resent 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 greenhouses. Such stems are 

 sometimes called condensed stems. 



79. 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, suc- 

 culent leaves. It is very difficult 

 to kill because of its power to con- 

 serve water. The houseleek, live- 

 forever, stonecrops, etc., are other 

 examples. 



80. Leaf -like stems or phylloclades. In these the leaves 

 are reduced to mere bracts, and the stem or branches of it are 

 broadened and flattened so that they resemble leaves. The 

 gardener's " smilax " (Myrsiphyllum}, so commonly grown in 

 greenhouses, is a good example. The main stem is slender and 



Fig. 47- 

 Phylloclades of smilax. 



