2(8 THE PLANT COVERING OF OCRACOKE ISLAND. 



Factors in the iDlij^sical environment of sand-strand vegetation 

 which tend to accelerate transpiration from the surface of the jDlant, 

 and hence contribute to the necessity for xerophytic structure, are : 



1. Exposure without shelter to the almost continual and often vio- 

 lent movement of air currents, which keep the plant's atmospheric 

 envelope constantly changing and prevent it from approaching a con- 

 dition of saturation. 



2. Intense light, both direct and reflected from the surface of the 

 sand. Light, which becomes converted into heat in the chlorophyll 

 tissue, increases transpiration ^ in proportion to its intensity. Besides 

 this effect of light, its direct and harmful action, when too intense, 

 upon the chlorophyll is to be guarded against, and this is probably 

 effected bj^ some of the modifications which also serve to reduce 

 transpiration. But in the present state of our knowledge it is imj)os- 

 sible to discriminate between the respective modifications Avhich 

 protect the plant against these two effects of light. 



3. Great heat during a great part t)f the year. Much more intense 

 than the atmospheric heat is that which is absorbed by and reflected 

 from the superficial layer of sand.^ 



It is probable, however, that the i)resence of a high percentage of 

 sodium chlorid in the substratum is at least as effective as any of 

 these causes in bringing about xerophytic structure. This is evident 



of the solution. Whether this is equally true of halophilous species is not 

 established. 



L. Diels ( Jahrb. Wiss. Bot. vol. 23, p. 316) doubts that osmosis in plants of saline 

 soils is sufficiently reduced to account for the absence of accumulations of salt to 

 an injurious extent in the tissues. He found that salt-marsh plants when trans- 

 ferred to distilled water showed a steady loss of salt from day to day, although the 

 impossibility of an excretion of the sodium chlorid as such could be demonstrated. 

 This author gives a number of analyses of halophilous species which would indi- 

 cate that in plants of that character the cells are enabled to decompose the 

 accumulated sodium chlorid, the sodium probably uniting with malic acid, while 

 the chlorin possibly combines with water and passes off through the roots as 

 hydrochloric acid. It is known that xerophytic modifications which protect the 

 plant against excessive transpiration at the same time cause an increased evolu- 

 tion of tree acids (notably malic acid) in the green tissue, by preventing the 

 ready access of oxygen and otherwise hindering the exchange of gases between 

 the plant and the atmosphere. These researches of Diels, if confirmed for halo- 

 philous plants generally, will prove an insuperable objection to Schimper"s theory 

 that such plants can prevent an indefinite accumulation of sodium chlorid in 

 their tissues only by reducing root action and hence transpiration. If we accept 

 Diels's conclusions, v/e should have to refer the xerophytic structure of halophilous 

 plants largely to its efficacy in preventing a f re-3 exchange of gases between plant 

 and atmosphere, thus rendering imperfect the combustion of carbohydrates in the 

 plant tissues and occasioning the production of considerable quantities of organic 

 acids, which serve the plant by decomposing the absorbed sodium chlorid. 



'Wiesner, Untersuch., p. 506. 



2 Volkens (Fl. .^gypt., p. 14) found a difference of from 22^ to 24° C. between the 

 temperatures of the surface soil and of the atmosphere in the shade near Cairo, in 

 Egypt, the maximum heat of the sand being 55° C. 



