MAN AS A MAKER OF NEW PLANTS — ANDERSON 475 



my Michigan boyhood, how pokeweed came in when a woodland near 

 our home was lumbered over. We had never noticed this weed in 

 that community, but the birds had been planting it wherever they 

 roosted. When the felling of the big oaks tore lesser trees up by the 

 roots, pokeweed plants appeared as if by magic for the next few 

 years in the new craters of raw soil. Man and the great rivers are in 

 partnership. Both of them are upsetters. Both of them breed weeds 

 and suchlike organisms. The prehuman beginnings of many of our 

 pests and fellow travelers are to be sought in river valleys. River 

 valleys also must have been the ultimate source of some of the plants 

 by which we live : gourds, squashes, beans, hemp, rice, and maize. 



The examples of the salvias and irises show how quickly evolution 

 through hybridization can breed out something new and different 

 under man's catalytic influence. What we should most like to know 

 is the extent to which weeds and similar organisms, created or at least 

 extensively modified through man's influence, are built up into whole 

 associations. It is clear that such things can happen ; the maqui vege- 

 tation of the Mediterranean, the skiblyak and karst vegetation of the 

 Balkans, the carbon scrub of Central America, are obviously very 

 directly the results of man's interference. One would like to analyze 

 the dynamics of these associations. We must do so if man is to under- 

 stand his own past or to be the master of his own future. For such 

 purposes we need ways of studying vegetation which are analytical as 

 well as merely descriptive — methods not based upon preconceived 

 dogmas. I should like to suggest that the methods used in analyzing 

 the Iris hybrids and the Salvia hybrids, if combined with other experi- 

 mental techniques, would allow us to get a long way into these prob- 

 lems. Let me illustrate what I mean by describing some recent studies 

 of Adenostoma, a fire-resistant shrub, which is a common component 

 of the California chaparral (Anderson, 1954) . 



Between the Great Valley and the Pacific Coast, Adenostoma 

 faseiculatum is one of the commonest shrubs in the California land- 

 scape. Noting that it varied conspicuously from one plant to the 

 next, I made collections of it near Palo Alto and applied to them the 

 methods of pictorialized scatter diagrams and extrapolated corre- 

 lates. The details of these techniques need not concern us here, since 

 they have been adequately published elsewhere, both in technical 



Figure 6. — Occurrence of pokeweed in two different habitats. Pokeweed {Phytolacca 

 americana) is an example of a species which is apparently native in the open soil along 

 American rivers but a weed in the open soil of disturbed habitats. (Map from Sauer, 

 1952.) Small dots represent single plants. Large dots represent five plants. It will 

 be seen that the pokeweed is occurring in two quite different kinds of habitats: in the 

 raw soil of repeatedly flooded woodlands on the immediate banks of the river and as a 

 weed around farm buildings, gardens, and the like. (See Sauer, 1952, for further details 

 and discussion.) 



