ORGAxNIC EVOLUTION 145 



pith or medulla), and each is divided by a thin sheet of 

 growing tissue known as cambium. Cambium divides the 

 stem as a whole into outer and inner portions that are 

 familiar to every one who has peeled a rod or made a willow 

 whistle ; we know them as bark and wood. The cambium 

 divides each bundle into an inner woody portion {xylem), 

 containing the open vessels for conduction of water, and an 

 outer bast portion {phloem) containing the sieve tubes, etc. 

 There is scarcely any development of bast fibres in the 

 chickweed, and the water conducting elements of the xylem 

 are spiral vessels. 



Cambium.^The most important new feature is this incon- 

 spicuous growth layer that divides the bundles. It forms 

 a sheath of thin cells that are rich in protoplasm and that 

 have retained their capacity for further division. Cell 

 increase in the fern stem may occur only at the stem apex. 

 When the stem is once formed and when its constituent cells 

 are fully grown, no further increase in its diameter is possi- 

 ble, but the cambium makes possible a continuance of 

 stem growth. Hence the plants that dominate the earth by 

 reason of their size, the trees of the forest, all have this 

 means of perennial growth. 



The cambium adds new cells during each growing season 

 to each of the layers it separates, and the growth of these 

 cells stretches the bark when it is young, and when it is old 

 and inelastic, cracks it and furrows it, or causes it to shed in 

 strips. 



Wood. — Increase of size of aerial plant body necessitates 

 increase of supporting structures, for the long reach of a 

 stem into a position favorable for getting sunlight would be 

 of no use unless the position could be maintained. Rigidity 

 of stem in plants having the manner of growth we have just 

 been describing is secured by further development of the 

 woody elements of the vascular bundles, by increase in 



