THE MICB.OSCOPE IN HISTOLOGY AND BOTANY. 131 



ing resinous and other matters. Woody fibre or ligneous 

 tissue is quite similar, save that the cells have become 

 elongated or fusiform, and when completely filled up with 

 internal deposit, fulfil no other purpose than that of me- 

 chanical support (Plate VII, Fig. 97). The woody fibres 

 of the Coniferce exhibit peculiar markings, which have 

 been called glandular (Plate VII, Fig. 98). In these the 

 inner circle represents a deficiency of deposit as in other 

 porous cells, while the outer circle is the" boundary of a 

 lenticular cavity between the adjacent cells. This ar- 

 rangement is so characteristic as to enable us to determine 

 the tribe to which a minute fragment, even of fossil wood, 

 belonged. 



Spiral Vessels. If spiral cells are elongated, or coalesce 

 at their ends, they become vessels, some of which convey 

 air and some fluid (Plate VII, Fig. 99). As in cells, the 

 want of continuity in the spiral fibre sometimes produces 

 rings, when the duct is called annular. In other instances 

 the spires are still more broken up by the process of growth, 

 so as to form an irregular network in the duct, which is 

 then said to be reticulated. A still greater variation in 

 the deposit produces dotted ducts. Not infrequently we 

 find all forms in the same bundle of vessels. 



Laticiferous Vessels (Plate VII, Fig. 100). These con- 

 vey the milky juice or latex of such plants as possess it, 

 as the Euphorbiacese, india-rubber plant, etc., and differ 

 from the ducts above described by their branching, so as 

 to form a network, while ducts are straight and parallel 

 with each other. 



The laticiferous vessels resemble the capillary vessels of 

 animals, while the spiral ducts remind us of the trachea 

 of insects. 



Siliceous Structures. The structures of many plants, 

 especially the epidermis, often become so permeated with 

 a deposit of silica, that a complete skeleton is left after 

 the soft vegetable matter is destroyed. The frustules of 



