54 OUR WOODLAND TREES. 
or tubes. But the walls of these are thinner than 
those in wood tissue, and they enclose long vessels, 
or spiral threads as they are called. The walls 
which enclose and make the cavities which are 
called cells—whether the latter are roundish as in 
cellular tissue, or oblong as in the tubes of wood 
or vascular tissue—consist, as we have seen, of a 
substance called cellulose, a substance compounded 
of carbon, hydrogen, and oxygen. The enclosing 
mass of protoplasm or the viscid material, which 
forms the cell contents, consists of carbon, hydro- 
gen, nitrogen and oxygen. 
The green colour which gives so much of their 
beauty to plants, is caused by the presence within 
the superficial cells of their tissue of minute 
granules of a matter called chlorophyll. These 
granules exist in great abundance, and can be 
seen so clearly through the thin transparent sur- 
face walls of the superficial cells that they give 
their familiar hue to all the green parts of a plant. 
The beauty of colour, too, in the bloom of plants 
is caused by the presence of other granules of 
colouring matter—granules which owing to their 
varied distribution throughout the flower petals 
