i62 General Botany 



a white carnation or a chrysanthemum. Repeat, using dilute 

 ammonia instead of red ink. 



Accumulation of water. The stems of many plants are suc- 

 culent ; that is, they accumulate water. Numerous examples 

 might be cited among desert plants of the cactus type, but they 

 also occur among mesophytes ; for example, purslane, begonia, 

 and certain orchids. Water retention depends in some instances 

 upon the presence in the cells of substances like pectic compounds 

 and mucilage, in other cases upon high osmotic pressures ; but for 

 still other cases no explanation can be given at the present time. 



The flow of sap. The water in stems may contain a small 

 amount of sugar in addition to mineral salts. In the maple, 

 during early spring when the days are warm but at night freezing 

 still occurs, quantities of sugar pass into the water-conducting 

 tissue. This sugar comes from the medullary rays and other 

 tissues where it accumulated in the form of starch during the 

 preceding growing season. With the coming of warmer weather 

 the starch is changed to sugar and diffuses into the water-con- 

 ducting vessels. 



The earlier sap is the richer and apparently comes largely from 

 the upper parts of the trunk. The last sap is more dilute and 

 probably comes from the roots. The positive pressure that pro- 

 duces the flow occurs usually during the day but may occur during 

 warm nights. When the temperature falls to the freezing point 

 at night, the pressure becomes negative and the sap flow ceases. 

 The causes of the pressure are only partly known. A portion of 

 it is due to the expansion of gas bubbles within the tree, but this 

 gas expansion accounts for only a small part of the pressure. 



Whether the flow shall continue for weeks or stop after a few 

 days is determined by weather conditions ; . but just how the 

 several weather factors (like changes in temperature or rainfall) 

 bring about the increase and decrease of pressure, is unknown. 

 The flow continues longest when the night temperatures are below 

 the freezing point, and the day temperatures above. Even under 



