PTERIDOPHYTES 89 



54. The vascular system. In all independent sporo- 

 phytes there develops a tissue which does not appear in the 

 dependent sporophytes of Bryophytes. It is called vascular 

 tissue, which means a tissue composed of vessels. The 

 so-called vessels are thick-walled, tubular cells that extend 

 through the sporophyte and are equipped to conduct water. 

 The vascular tissue does more than conduct water, but its 

 other work will be considered later. Of course water is 

 conducted through the bodies of Liverworts and Mosses, 

 but the vascular tissue conducts it with more rapidity and 

 precision than any other tissue. The difference between 

 water-conduction in a liverwort and in a plant with vascular 

 tissue may be likened to the difference between water work- 

 ing its way through a swamp and water moving in definite 

 channels. 



It must not be supposed that the water-conducting vessels 

 are continuously open tubes, as are the arteries and veins of 

 the human body or the water pipes of a house. They are 

 elongated cells set end to end, so that water in moving through 

 the tissue must pass through numerous cell-walls, thousands 

 of them in an ordinary stem. How water moves under these 

 conditions is not known with certainty, but the direction of 

 its movement is clear. 



The vascular tissue does not extend at random through the 

 body of the sporophyte, but has a definite organization, so 

 that there is a vascular system in every sporophyte. The 

 vascular system has proved to be of very great service in the 

 study of the relationships of vascular plants, for it differs 

 in the various great groups. That part of the vascular tissue 

 which conducts water is called the xylem, which means. 

 " wood," for the ordinary wood of trees is xylem tissue. 



55. The leaf. In addition to a vascular system, the 

 independent sporophyte has leaves. Leaves are simply ex- 

 pansions of green tissue that increase the amount of green 

 tissue exposed, and so increase the capacity of the plant for 



