366 THE VASCULAR TISSUES AND TRANSPIRATION. 



A particularly noticeable method of conducting water from the soil to the green 

 leaf -blades is exhibited by some large-leaved tropical Avoids which climb up trees, 

 and are provided with aerial roots. These plants have really two kinds of aerial 

 roots, viz.: shorter ones, which are at right angles to the stem, by means of which 

 they climb up their support, usually old tree-trunks; and longer ones, passing 

 down perpendicularly to the ground like ropes or strings. In the Mexican 

 Tomelia fragrans (Philodendron pertustvm) represented in fig. 96, these latter 

 roots attain a length of 4-6 metres and a diameter of 1-2 cm. They are of 

 uniform thickness, brown, smooth, unbranched, and quite straight. As soon as 

 they reach the ground, the tip bends round almost at a right angle, and sends 

 a number of lateral roots which are covered with an actual fur of root-hairs into 

 the soil. The bent end then enters the soil for a short distance, and thus the 

 entire aerial root is rendered fairly tense. As a rule, two such cord-like aerial 

 roots originate below each new leaf, and it seems as if this arrangement was 

 specially adapted to transport the necessary food-sap from the soil to the large 

 luxuriant leaf above by the shortest path. But it not only seems so, for this is 

 actually the case, and it is especially remarkable that root-pressure takes a 

 prominent part in the transport. On cutting through one of these cord-like aerial 

 roots about a span above the ground, watery fluid is immediately seen to ooze from 

 the middle of the cut surface. The woody portion of the root, which here forms a 

 central strand, contains very wide conducting tubes, like those in the stems of lianes, 

 and the quantity of fluid exuded in thirty-six hours amounts to as much as 17 grms. 

 It is noteworthy that the root-pressure here, according to all appearances, acts with 

 the same force all through the year. In the vine this is not the case. Vines. which 

 are cut through in the summer, it is well known, no longer weep; the cord-like 

 aerial roots of tropical aroids, on the other hand, weep at all seasons of the year 

 when cut across. Indeed, the vegetative activity is never entirely interrupted in 

 these plants all the year, and it should be remembered, in connection with this fact, 

 that they grow in places where the air and soil are always warm, and where their 

 humidity is only subject to slight variations. It may happen that in damp, warm 

 places transpiration from the leaves ceases for a time entirely, and then it is very 

 necessary that the amount of food-sap should be forced up to the leaves by root- 

 pressure in order that they may be supplied with the food-salts they require. The 

 water, which contained dissolved food-salts, is of no use when it has given these up, 

 and it is therefore forced out of the stomata, these in consequence being trans- 

 formed into water pores. 



The aerial roots, which form the shortest and straightest channels for con- 

 ducting the raw food-sap to the leaves, are, moreover, of great importance to these 

 tropical aroids, since it not infrequently happens that the lower portion of the 

 stem' in an old plant dies off, leaving the upper part, which is fastened to the 

 trunk of a tree by the earlier-mentioned short supporting roots, and therefore in 

 no direct connection with the ground. The supporting roots would not be sufficient 

 to supply the fluid food required, and the whole plant is therefore provided 



