ELEMENTS OK MUUCTflUL HolANY. 100 



hc.rk, and the consequent toughness conferreil upon 

 that part, the tissue formed by them is specially dis- 

 tinguished as lutst tissue. Associu'ted witli the wood- 

 cells are commonly fouud others, differing fron fhom 

 chiefly in being larger in diameter, and firmed out of 

 rows of short cells, standing end to end, by the disap- 

 pearance of the partitions which separated thera. 

 These enlarged cells, produced in this way, arc - 

 called cessfls or ducts, and a combination of | 

 them is known as vascular tissue Ducts in- r •, 

 variably show markings of some sort on their ) ^ -i 

 walls. The one figured in the margin (Fig. \:--'-^ 

 IGG) is a dotted duct, the dots being spaces i- 

 which have not been thickened by deposits of ' 

 cellulose. Other ducts are sjiiniUij marked on f 

 their inner surface, but in this case the mark- i- 

 ings are themselves the thickened part of the pig. inr. 

 cell-wall. It is convenient to speak of the mixtur>. uf 

 woody and vascular tissue as the jilno-rascular si/strni. 



The name jianiK fr/iua is commonly applied to ordi- 

 nary cellular tissue, whilst tissue formed of long cells 

 is callod prosoii fiifuia. 



It will be understood, then, that all cells of every 

 description, found entering into the composition of a 

 plant, are only modifications of ono original form, the 

 particular form ultimately assumed by the cells depend- 

 ing mainly on the functions to bo discharged by that 

 portion of the plant in which the' cells occur. 



e::ogenous and endogenous stem.--. 



1G8. It has already been hinted that the two great 

 classes of plants. Dicotyledons and Monocotyledons, 

 differ in the mode of growtli of tiieir stom.s. ^Vc sliaJl 



