DIGESTION IN PLANTS 91 



roots. On the inside is found the xylem (zi'lem), or woody tubes 

 that carry water upward. 



In the monocotyledonous stem the bundles are scattered, lack 

 the cambium, and increase in number as the stem grows older. 

 They contain sieve tubes on the inside and water-bearing tubes 

 in their outer part. 



What causes Water to rise in a Stem. — • We have already seen 

 that osmosis is responsible for getting water inside the root, and 

 that the pressure exerted by this water {root pressure) is frequently 

 capable of forcing fluids a considerable distance up a living stem. 

 But during most of the year root pressure plays a very unimportant 

 part in this phenomenon. It has been found that in small tubes, 

 such as we find in wood, the cohesive force of molecules of water is 

 very great. Also a very large amount of water is evaporated every 

 day through the stomata. This, according to Ganong, averages 

 about 50 grams per square meter of leaf surface in daylight and 

 about 10 grams in darkness, almost half a ton of water being evap- 

 orated from a large tree on a warm summer's day. This evapora- 

 tion causes a pull on the volume of water in the fibrovascular 

 bundles and is an important factor in the rise of fluids in stems. 



Digestion. — Much of the food made in the leaves is stored in 

 the form of starch. But starch, being insoluble, cannot be passed 

 from cell to cell in a plant. In our study of the root hair we found 

 that substances in solution {solutes) will pass from cell to cell by 

 diffusion. In our study of a growing seedling we found that a 

 solid food substance, starch, was digested in the corn grain by an 

 enzyme, thus becoming a diffusible substance which could pass 

 from cell to cell. This process of digestion seemingly may take 

 place in all living parts of the plant, although most of it is done in 

 the leaves. In the bodies of all animals, including man, starchy 

 foods are changed in a similar manner, but by other enzymes, into 

 soluble grape sugar. 



The food material may be passed along in a soluble form until 

 it comes to a place where food storage is to take place, and then it 

 can be transformed again by the action of a reversible enzyme into 

 an insoluble form (starch, for example) ; later, when needed by the 

 plant in growth, it may again be transformed and sent in a soluble 

 form through the stem to the place where it will be used. 



