Vessels 23 



with one another ; when they do so it is by means of their 

 lateral branches. 



Very closely related to the sieve-tubes mentioned above, 

 especially in containing like them nutrient fluids, are vesicular 

 vessels and laticiferous vessels. The former consist of elon- 

 gated cells containing a milky juice and bundles of needle- 

 shaped crystals. 



Laticiferous vessels are tubes more or less branching, 

 often forming a complete network, and containing a fluid 

 known as latex, which is often milky, sometimes coloured, and 

 varies in its composition in different plants. 



Fig. 40. — Transverse section of phloem of root of Scorzonera hispanica., 

 showing branching and anastomosing laticiferous vessels. 



CeUs do not grow indefinitely ; the size of the adult cell 

 varies. Prosenchymatous cells vary from ^^ to yV of an inch 

 in length, and from -^-^ to ^-^^ of an inch in diameter. 

 Parenchymatous cells vary as a rule from ^i^ to TaVu °f ^" 

 inch in diameter, whilst some in the pith of plants, in suc- 

 culent parts, and in water plants, are as much as -^^ or even 



Jj- of an inch in diameter. 



As the plant increases in size, fresh cells are produced, 

 being formed out of those already existing. This process is 

 know as cell formation. There are several modes by which 

 this takes place, but in the formation of the vegetative cells of 

 higher plants it is always by means of cell division. Vitally 

 active cells always contain a portion of the protoplasm more or 

 less distinctly separated from the general mass, known as the 

 nucleus, and which often contains small granules or nucleoli. 



