34 THE TISSUES OE PLANTS 



elongated utricles become lengthened into tubes, and the 

 septa or partitions which separate them, and which in this 

 instance are not absorbed, assume an oblique position with 

 reference to their interior. 



The diiferent -varieties of vasiform tissue or ducts will 

 be seen to originate like the woody fibre from a row of 

 elongated utricles; but the membrane which forms the 

 walls of the fibre cells appears to be more susceptible of ex- 

 tension than that of the duct cells. Both the transverse 

 and lateral walls of the row of utricles which form the fibre 

 cells are elongated, and this takes place owing to the great 

 tenacity and extensibility of the walls of the several 

 cellules, without any rupture or open communication be- 

 tween them being efiected. Hence it is that the fibre cells 

 overlap each other at their extremities, or are as it were 

 spliced together, and their calibre or bore must necessarily 

 diminish in proportion to the degree of their attenuation. 

 It is in fact reduped to the very finest degree of capillarity, 

 so that the tubular character of the fibre cells can only be 

 verified by employing the very highest powers of the best 

 microscopes. The parietes of the row of utricles which 

 originate the several varieties of vasiform tissue or duct 

 cells, on the other hand, will not submit to a similar degree 

 of tension ; on the contrary, a very slight degree of elonga- 

 tion is sufficient to rupture the cross walls of the several 

 cells, so as to form a continuous communication between 

 them. An uninterrupted tube with a conspicuous calibre 

 or bore, is the natural result. These ducts are generally 

 situated on the inner side of the circle of fibre cells, and 

 their open mouths are not unfrcquently visible on the cross 

 section without the aid of the microscope in the form of 

 rounded openings or pores. 



In their earliest condition, the cells of animals, Uke those 



