MAN AS A MAKER OF NEW PLANTS — ANDERSON 473 



like radishes or lettuce. You will be amazed to learn how small a per- 

 centage of them ever comes up at all. Make similar collections from 

 the weeds in a vacant lot or from the plants (wanted and unwanted) of 

 your garden. Nearly all of them will come up promptly and grow 

 readily. Where did these open-soil organisms come from in the first 

 place, these weeds of gardens and fields, these fellow travelers which 

 rush in after the bulldozer, which flourish in the rubble of bombed 

 cities ? Well, they must have come mostly from prehuman open- soil 

 sites. River valleys did not supply all of them, but rivers are cer- 

 tainly, next to man, the greatest of weed breeders. Our large rivers 

 plow their banks at flood times, producing raw-soil areas. Every river 

 system is provided with plants to fill this peculiar niche; all those 

 known to me act as weeds in the uplands. One of the simplest and 

 clearest examples is our common pokeweed, Phytolacca americana, 

 native to eastern North America. It will be found growing up abun- 

 dantly in the immediate valleys of our major rivers (Saner, 1952; 

 see fig 6). On the uplands it is strictly limited to raw soil, though, 

 once established in such a habitat, it can persist vegetatively for a 

 long time while other kinds of vegetation grow up around it. Being 

 attractive to birds, its seeds are widely scattered. I remember, from 



I. fulva !. hexagona giganti-casrulea 



Hybrids 



Figure 5. — Sepals and petals of some hybrids of Iris hexagona giganti-caerulea and I. fulva, 

 somewhat diagrammatic but accurately to scale. In each case the sepal (the so-called 

 "fall" of iris fanciers) is shown to the left; the petal, "standard," to the right. I. fulva 

 has small lax terra cotta sepals and petals. /. hexagona giganti-caerulea has large crisp 

 petals and sepals of bright blue. The sepal has a brilliant yellow signal patch (shown 

 in black) surrounded by a white area (shown by stipples) shading off into the blue. Note 

 that in the various hybrids the small-sized flowers (characteristic of I. fulva) tend to be 

 associated with the lack of a white area (another fulva characteristic). Note the varia- 

 bility of the hybrids. In color they varied from deep wine to very pale light blue. 



