ADJUSTMENT TO WATER 51 



tion of absorbing and transpiring surfaces is effected by the petiole. 

 By far the greater bulk of the water absorbed must be carried 

 through the stem to the leaves. The rapid movement of the large 

 amount of water which is lost by the active evaporation from the 

 leaves makes the presence of a special pathway imperative. Such 

 a pathway is found in the fibrovascular system, which has already 

 been seen in the root to occupy a position peculiarly advantageous 

 with respect to the diffusing water. The fibrovascular bundle 

 of the root connects directly with those of the stem, and the bimdles 

 of the stem are prolonged in the form of minute veins to all parts 

 of the leaves. The fibrovascular system thus resembles a con- 

 tinuous series of water pipes, serving to collect the water absorbed 

 by the roots, to carry it with relatively slight loss through the 

 stem, and to distribute it to all parts of the leaf. This movement 

 of the water, which is an essential function of stems, is transport. 

 The details of transport will be more readily understood after a 

 consideration of the structure of stems. 



65. Types of stem structure. Among flowering plants, dicoty- 

 ledons possess a stem essentially different in structure from that 

 of monocotyledons. Though in less degree, woody stems also 

 differ characteristically from herbaceous ones. All of these types 

 are in fundamental agreement, inasmuch as each possesses the 

 three primary regions corresponding to dermatogen, periblem, and 

 plerome, though in monocotyledons the regions are confused. 

 The essential differences between the three kinds of stem hinge 

 upon the nature and arrangement of the fibrovascular bundles. 

 The bundles of a dicotyledonous stem possess a layer of meristem, 

 or cambium, and are able to increase their size. They are usually 

 arranged in a more or less circular row placed a]:)Out midway 

 between the center and the surface of the stem. The bundles of 

 monocotyledons have no cambium, and are consequently unable 

 to grow after they are once formed. They are scattered more or 

 less uniformly throughout the stem. The bundles of fern stems 

 are also without cambium, and are solitary or scattered. Woody 

 stems are primarily dicotyledonous or monocotyledonous. They 

 differ from herbaceous stems of the same type in the excessive 

 development of woody tissues. This is shown in oui' coiniiion 

 shrubs and trees, all of which are dicotyledons. In llicsc^ ihc 

 number of bundles is repeatedly augmented by the formation of 

 new bundles from the ring of cambium, until the filirovnsculnr 



