1912] STARR— ANATOMY OF DUNE PLANTS 301 



A summary of stem characters is as follows : 



Vessels more numerous 14 X- 7 M (1 same) 



Vessels larger 9 X-n M (2 same) 



Total area larger 17 X- $M 



Walls of vessels heavier 16 X- 4 M (2 same) 



Walls of fibers heavier 14 X- 6 M (1 same) 



Lumen of fibers smaller 16 X- 2 M (2 same) 



More growth rings 10 X- 6 M (3 same) 



More sclerenchyma and collenchyma 15 X- 6 3f (1 same) 



Cork thicker 9 X- 8 3f (1 same) 



There is a tendency for the vessels to be larger in the mesophytic 

 forms, but more numerous in the xerophytic, the area being greater 

 m the xerophytic. A greater number of xerophytic forms have 

 heavier walls of vessels and fibers and smaller lumen in the fibers, 

 making a more woody cylinder. A majority of xerophytic forms 

 have more growth rings to the given diameter than the mesophytic 

 forms, showing slower growth under the more adverse conditions. 

 A majority of xerophytic forms show an increase in mechanical 

 tissue as well as in the wood and an increase in cork formation, 

 though this is not so marked as one might expect. The internodes 

 m the stem, in every case measured, were shorter in the xerophytic 



form. 



the 



seem more aDt than the 



general stem situation. Their vessels are always extraordinarily 

 large, but why they should often be larger and more numerous in 

 mesophytic forms, when those of trees are not, is impossible to guess. 



Discussion of theories 



hich 



the plants live, or have lived in the past, is undoubted, but what 



mmediate 



The 



purpose of this investigation has been to get at a few facts, but it 

 may be of some interest to review a few of the theories: 



Mrs. Clements (4) considered light the principal factor in 

 the development of deeo Dalisade. Haberlandt (16) said that 



disposition, and 



this 



