48 PLANT PHYSIOLOGY 



natural that in 90 per cent of all plants the seeds belong to the 

 oily type. The only advantage possessed by reserve carbo- 

 hydrates lies in the fact that without any complex chemical 

 changes they can be converted into glucose and monosaccharides 

 in general, substances of ready availabihty and great usefulness 

 to plants, while fats must undergo many transformations before 

 they can be utilized. 



A group of substances closely related to the fats are the 

 lipoids, or fatlike substances. The most important representa- 

 tive of this group is lecithin, a complex ester composed of glycerin, 

 two fatty acids, and phosphoric acid, which in its turn is united 

 with a strong, nitrogen-containing, organic base, choline. The 

 general formula of lecithin is: 



(CH3)3 



CH2(OOCR)CH(OOCR)CH20PO(OH)OC2H4N 



\ 

 OH 



in which RCOO represent the various fatty acid radicals. 



Lecithins are found in all living cells, animal as well as plant. 

 They are not reserve substances but enter into the composition 

 of the protoplasm as a necessary integral part of it. There is 

 reason to believe that they accumulate especially in the so-called 

 "plasma membrane." Overton, as has already been noted in 

 Art. 7, attributes an outstanding function to them in the deter- 

 mination of the osmotic properties of this membrane. Some 

 authors, Palladin, for instance, attribute to lecithins and to 

 other closely related substances, the so-called '^ phosphatides," 

 an important role likewise in the oxidation processes during 

 respiration. Finally, having the capacity to form easily complex 

 compounds with a great variety of substances, particularly the 

 proteins, lipoids may have an important role in the synthetic 

 processes of the cell. All these, however, are only suppositions 

 of a hypothetical nature. There are no exact experimental data 

 that would indicate the significance of lipoids and phosphatides. 



Closely allied to fats are waxlike substances, similar to fats in 

 their fatty consistency and solubility in ether. They represent 

 complex esters of fatty acids and monatomic higher alcohols, 

 more often of ceryl, cetyl, and mericyl alcohols, and likewise of 

 a cyclic alcohol phytosterol. Waxlike substances are very 



