CELLULAR TISSUES. 



333 



Fig. 175. — Stellate tissue, from stein 

 of a Ilzish. 



Fig. 175, in the stellate tissue cut from the stem of a 

 rush, we have the forma- 

 tive network dividing into 

 ducts for the purpose of 

 giving strength and light- 

 ness to the stem of the plant. 

 These ducts may undergo 

 other transformations ; the 

 cell itself become gra- 

 dually changed into a 

 spiral continuous tube or 

 duct, as seen in fig. 198 ; 



these are sometimes formed by the breaking down of the 

 partitions; in the centre of which we may have a com- 

 pound spiral duct, resembling portions of tracheae from 

 the silkworm. 



Another important change occurs in the original cell, — 

 it is that of its conversion into woody fibre. Common 

 woody fibre {Pleurenchymd) 

 has its sides free from de- 

 finite markings. In the 

 coniferous plants, the tubes 

 are furnished with circular 

 discs ; these discs are 

 thought to be contrivances 

 to enable the tubules of 

 the woody tissues to dis- 

 charge their contents from 

 one to the other, or into the 

 cellular spaces. Plants having aromatic secretions are 

 furnished with glands ; these form a series of interesting 

 objects, and such as the sage-leaf should be mounted as 

 opaque specimens. A large central gland is seen in a section 

 of a leaf from Ficus elastica, India-rubber- tree, fig. 177, ISTo. 2. 

 Professor Quekett observes, " The nature of the pores, or 

 discs, in conifers, has long been a subject for controversy ; 

 it is now certain that the bordered pores are not peculiar to 

 one fibre, but are formed between two contiguous to each 

 other, and always exist in greatest numbers on those sides 

 of the woody fibres parallel to the medullary rays. They 

 are hollow ; their shape biconvex ; and in their centre is 



Fig. 176.-^4 section of stem of Clematis, 

 with pores, highly magnified, to show 

 the line which passes round them. 



