498 



VASCULAE TISSUE. 



reason of this quality of its primordial utricle, or through un- 

 Vascular tissue * 



and its modifi- equal nutrition, or other cause, gives origin to a tube. And 

 if, of several cells thus elongated, and placed end to end on 

 each other, the terminal portions should be obliterated either by rup- 

 ture or absorption, a vessel permeable throughout is the result. In this 

 manner vascular tissue arises. These vessels still exhibit the structural 

 peculiarity of the cells from which they have originated in this, that they 

 may be fortified in their interior with fibres wound in a spiral, and so 

 constituting a spiral vessel ; or wound in rings, and forming annular 

 ducts. In like manner, through similar modifications, the varieties known 

 as reticulated and dotted ducts arise. In these fibre-vascular tissues it 

 frequently happens that the fortifying thread is double or even quadru- 

 ple. Of spiral vessels derived from a cactus we have an example in the 

 photograph, Fig. 235, and in those from the banana in that of Fig. 236. 



Fig. 236. 



Fig, 235. 



Spiral vessels of cactus, magnified 50 diameters. 



Spiral vessels of banana, magnified 50 dinmctcrs. 



The spiral vessels of plants Contain air. Other tubes are for the 

 Spiral vessels, conveyance of liquid; the laticiferous vessels, for example, 

 v "f'co- which are branching tubes Fi ,. 237 . 



nifers. for transmitting the latex 



of plants. Again, in other cases, the 

 interior of the vessel is more or less 

 completely filled up by a gradual de- 

 posit of solid material, it being in 

 this manner that proper woody fibre 

 is formed from long, spindle-shaped 

 cells. Vascular tissue in coniferous 

 plants presents a peculiar dotted, as- 

 pect from disc-like forms, exhibiting 

 a pair of concentric circles, which are 

 set at regular intervals upon it, as 

 shown in the photograph, Fig. 237, 



Woody fibre of pine, magnified 50 diameters. 



