536 
HERBERT C. HANSON 
peaty soil as causes for the xerophytic leaf structure of some* New 
Zealand plants. 
Raunkiaer (27) found that palisade tissue was equally well de- 
veloped in the leaves of certain plants above the water and to a depth 
of twenty centimeters. Below this the length of the paHsade cells 
gradually decreases. No palisade tissue was distinguished in shade 
plants under water, nor above, to a height of about thirty centimeters. 
His discussion of the causes of palisade development and the orienta- 
tion of the palisade cells is valuable. 
Lubimenko (25) determined that the chloroplasts of certain 
ombrophilous plants, as Tilia and Abies, were greater in size and in 
sensitivity than the chloroplasts of ombrophobous plants, as Pinus and 
Betula. The pigment was more concentrated in the former group. 
Baumert's (3) work contains a good review on the literature of 
structures protecting leaves from light. From his own experiments 
he found that a thick white coating of hairs as in Centaurea candidissima 
reduced the heat in the leaf 37.5 percent, shininess reduces the heat 
30 percent, and a wax coating up to 13.6 percent. Wiegand's (31) 
experiments show the efficiency of hairy and cutinized coverings in 
reducing the water loss by transpiration. 
Areschoug (2) maintains that well-developed, compact palisade 
tissue reduces transpiration, despite Hesselman's experiments to the 
contrary. 
Ewart (15) experimenting upon Tilia europaea found that mature 
leaves do not increase in size when most of the leaves are removed from 
the tree. The increased size of the new leaves which replace those 
defoliated is due to the increase in the number of cells. 
Sampson and Allen (29) found that the sun leaf transpired from 
two to four times as much as the shade leaf of the same species whether 
the leaves were placed in the sun or shade. This was explained by 
the greater number (20 percent-60 percent) of stomata in the sun leaf. 
Harshberger (20,19) investigated the leaf structure of strand plants 
in New Jersey, and sand dune plants in Bermuda. He states that the 
xerophytic structures are due to intense light, strong winds, and in 
rare cases to salt spray. The unequal illumination of the two sides of 
the leaf causes the formation of palisade and sponge tissues. 
Renner (28), in discussing the relation of wind to transpiration, 
says that the transpiration of small mature leaves is increased to a far 
greater extent by the wind than that of large leaves. This is ex- 
