168 BIOCHEMICAL SYSTEAAATICS 



are absent from algae and aquatic plants in general. In aquatic higher 

 plants water-soluble toxic products may be eliminated directly into 

 the environment without the requirement of detoxification mecha- 

 nisms. The aquatics which produce alkaloids are usually those with 

 floating leaves. For example, the submerged Ceratophyllaceae lack 

 alkaloids while the closely related Nymphaeaceae with floating leaves 

 are alkaloid containing. [It is interesting to note that in the genus 

 Cabomba (Nymphaeaceae), in which the leaves are mostly sub- 

 merged, alkaloids have not to our knowledge been reported.] 



Thus far there has been little or no evidence of physiological 

 effects of alkaloids upon the plants in which they occur. Dawson 

 (1948) has expressed skepticism that negative results effectively settle 

 the question. Alkaloids of one species may apparently affect other 

 species— even close relatives. Mothes (1960) noted that when bella- 

 donna or tomato was grafted onto Nicotiana stock, nicotine migrated 

 into the scion and browning occurred. The browning was assumed to 

 result from the presence of nicotine. 



General considerations of the 

 systematic value of alkaloids 



Previously, it was noted that the protoalkaloids and nicotine 

 had a distribution which suggested that parallel evolution accounted 

 for their presence in certain widely separated plant groups. Rowson 

 (1958) has noted that the distributions of anabasine, berberine, and 

 caffeine did not closely correlate with the systematic position of the 

 plants in which they occur. The same is true of 3-methoxypyridine 

 (found in Equisetum and Thermopsis), the barman alkaloid types, 

 and others. Since parallel evolution of morphological attributes is 

 also regularly encountered, similar parallelisms among biochemical 

 components should not be cause for excessive pessimism concerning 

 their use. Parallel evolution is likely to be responsible for many 

 possible misinterpretations of biochemical data. 



Within an alkaloid series it is probable that alkaloid com- 

 plexity is correlated generally with systematic advancement. However, 

 McNair (1935) somewhat naively correlated the molecular weights of 

 alkaloids with the Engler and Prantl family index number. The 

 "percentage of frequency rule" (Chapter 4) supports the previous 

 generalization but only within a closely knit group wherein parallel 

 evolution for the character is minimized. It is of course important to 

 estabhsh better criteria of complexity than merely molecular weight. 

 For example, the genetical basis for the synthesis of a bis-benzyliso- 



