62 NINTH REPORT. 



the iip-ciirreiit of the maple may be through portions of the wood which are 

 several years old, sometimes as many as eighteen or twenty. An important 

 exception to the system of which the sunflower has been taken as a type is 

 that which occurs in the pumpkin. Here the backward flow of organized 

 material ^msses, in part, through a portion of the tissue next the pith as well 

 as through the cortex. There are two general regions through which the 

 organized material flows backwards from the leaves towards the root, one on 

 the outside of the woody fibre, and the other on the inside. Through the 

 woody fibre, of course, the water from the soil passes in an opposite direc- 

 tion, just as it does in the maple or the sunflower. 



The corn plant differs materially in structure from the sunflower mainly 

 in that its conducting fibres are scattered (in cross section) throughout the 

 whole stem, while in the sunflower they are in one concentric ring. But these 

 two stems are similar in that the up conducting portion of the fibre is on the 

 side of the fibre towards the center of the stem, while the down conducting 

 ])art lies on the opposite side. It can scarcely be said that any one of these 

 four types of system is better adapted to the conducting of liquids than any 

 other, though from a mechanical point of view there might seem to be a 

 choice. The up system in the maple as compared w^ith the pine would no 

 doubt be capable of transmitting a greater amount of liquid in a given time, 

 but the leaves of the pine need (for a given weight of leaf substance) a great 

 tleal less water than the leaves of the maple. A maple requires about 75 

 liters of water where the pine requires only about 7 liters. 



One other special instance of conducting vessels should be mentioned. A 

 considerable number of plants produce a milky juice, e. g., the dandelions, 

 the dogbanes, the spurges. Such plants have a peculiar system of tubes 

 situated in the cortex. In these is found a sort of an emulsion composed of 

 oils, gums, water and other material, and sometimes starch grains. These 

 tubes are sometimes branched and often they anastomose, but they are 

 extremely long, forming a w^ell defined means of transportation of such ma- 

 terial as may be in them. The general course of the current in these tubes 

 is towards the root, and the material is likely of such a nature as may be 

 drawn upon by the plant for food when necessary. Some say that the latex 

 (material in these tubes) is not a reserve food, but the weight of evidence and 

 of probability is against such a conclusion. 



In general, there are special portions in leaves of plants which are adapted 

 to the liberation of water. When the stomates are inactive during the night 

 the stomatal openings are small or closed entirely, and as such forces in root 

 and stem are as active in transporting water as in the day time, there must 

 be some other means of outflow than stomates. Many plants have such 

 structures, and these are called water-pores. They occur commonly on the 

 tips of leaves of grasses, notabl}'' corn, wheat, oats, barle}^, June grass and the 

 like. They occur also near the ends of the chief veins or veinlets in such 

 leaves as grape, strawberry and nasturtium. On such plants small drops of 

 water may be seen on the parts mentioned, in early morning. This has been 

 called dew, but it is really water which has passed through the plant and con- 

 tains inorganic substances, as well as, occasionally, small amounts of organic 

 matter. It is not a deposit from the air as is generally supposed. 



Just as there are special portions of plants adapted to the liberation of 

 water, so there are also specialized parts adapted to check the flow of water. 

 Downy coverings, oily or w^ax}^ surfaces, sunken stomata are thought to be 

 such adaptations. 



A plant like the grape vine or Boston ivy which has a large number of 



