GENERAL EVALUATION 335 



variations in the pattern and nature of the distribution of certain 

 chemicals. 



In general, the more important a particular compound is to 

 the survival of the species, the more effectively deficient mutations are 

 eliminated. If the mutation frequency is quite high, however, equilib- 

 rium may be reached with a fairly high representation of the deficient 

 type in the population. Also, the more important the compound to the 

 survival of the species, the more likely the existence of indirect genetic 

 buffering mechanisms which tend to inhibit drastic changes in the 

 amount of the substance formed. This type of buffering can be effective 

 against intrinsic (genetic recombination) or extrinsic (environmental 

 factors) changes. Flower color in species with specific cross-pollinated 

 vectors may represent good examples of such a situation in which 

 pigment content of the petal is kept constant. Pigment content 

 of stems, in contrast, may be more variable. 



As noted in previous sections, although secondary substances 

 may vary significantly, basic metabolites such as amino acids may 

 vary as much or more, especially as a result of differing ecological 

 factors. Pertinent to this is the recent suggestion by Jabbar and 

 Brochmann-Hanssen (1961) that the geographical origin of opium 

 might be traced through an analysis of the amino acid composition 

 of the crude drug sample, the implication being that the amino acids 

 are more valuable, by virtue of being more variable in this case, than 

 are the alkaloids of the opium poppy. When deahng with systematic 

 categories above the species level, however, it is not likely that com- 

 mon amino acids will prove of much phyletic significance. Thus, 

 Erdtman (1956) considers the heartwood constituents to be the most 

 reliable compounds in biochemical systematics since they are deposited 

 over a long period of time as more or less metabolically inert substances. 



Variation in the course of development 

 and within the mature plant 



It is obvious that tissues of the same plant as physically 

 different as roots, stems, leaves, flowers, fruits, and seeds are physio- 

 logically, hence biochemically, distinctive. It should be equally 

 apparent that overt differences such as chlorophyll, carotenoid, and 

 anthocyanin pigment composition are matched by differences of a 

 more subtle nature involving other classes of compounds. For example, 

 alkaloids of leaves and stem of yohimbe differ (Paris and Letouzey, 

 1960); steroidal sapogenins of leaves and seeds of Agave differ (Wall 

 and Fenske, 1961); and similar examples utihzing other classes of 

 compounds could be cited. 



