224 TEXTBOOK OF BOTANY 



feet, six feet above the ground. Aside from conifers (includ- 

 ing the redwoods), probably the largest tree in the United 

 States is a sycamore at Worthington, Indiana, which is 

 about 150 feet high and 42 feet in circumference five feet 

 above the ground. The ages of various large trees have been 

 estimated at 4000 or 5000 years, and one famous tree, the 

 " dragon tree " growing on the island of Teneriffe (Fig. 135), 

 was said to be 6000 years old. Such estimates seem to be 

 much exaggerated. There is evidence that the giant red- 

 wood, the true cypress, and the yew may live about 3000 

 years, and that the chestnut, one of the oaks, and the cedar 

 of Lebanon may live 2000 years. Probably not much 

 greater ages have been reached by any plants now living. 



241. The Smallest Seed Plants. — These are members of 

 the genus Wolffia. The common Wolffias of the United 

 States and Canada consist each of a rounded green body 

 which floats in ponds and lakes and is so small (at most not 

 more than a sixteenth of an inch in diameter) that it may 

 easily be taken for a minute seed. The rounded mass of 

 cells is a stem, which branches by forming another small 

 group of cells within a sort of pocket. The branch breaks 

 away and becomes an independent plant. There are no 

 leaves or roots, and each plant may bear two simple flowers, 

 one consisting of a single stamen, the other of a single pistil. 



242. Grafting. — It has been known since very ancient 

 times that parts of two plants may be made to grow together 

 into what is practically a single plant. In most cases the 

 process of grafting consists in attaching a bud or small shoot 

 (called the scion) from one plant to a plant or to the lower 

 part of a plant of another sort, which is called the stock. 

 The advantage of grafting varies in different cases. Some- 

 times a cultivated plant bears no seeds or its seeds do not 

 germinate well, and it cannot easily be multiplied by other 

 vegetative means, such as by layerings or cuttings. Scions 

 of such a plant may be grafted upon stocks of less desirable 



