THE INTERNAL STRUCTURES OF PLANTS. 



207 



the broken, mutilated coils of spiral vessels. The 

 uncoiling, spiral thread is thus easily seen, but the 

 wall of the vessel is difficult to find. 



FIG. 875. 



FIG. 376. 



FIG. 377. 



These membranous threads of spiral vessels con- 

 tinue, without interruption, from one end of the ves- 

 sel to the other. In most cases they are simple, but 

 sometimes they are found double (Fig. 

 377), triple, etc. Even twenty juxtaposed 

 threads have been seen forming a ribbon, 

 and unrolling all together. Spiral threads, 

 that at first were simple, sometimes split 

 into several very fine threads. The spiral 

 thread is neither tubular nor channelled ; 

 it may be round, flat, or square. The 

 markings of annular vessels, like Fig. 377, 

 and vessels that have at the same time 

 both annular and spiral threads (Fig. 369), 

 reticulated, scalariform, and dotted vessels, 

 are produced in the same way as are simi- 

 lar markings upon cells, as explained on 

 page 199. " 



When you break a stalk or leaf of milkweed, let- 



