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The Plant World. 



desert or semi-desert surroundings. In this connection it will 

 be interesting to observe that thus far no native species belong- 

 ing to these groups of plants has been found growing during 

 both the winter and the summer periods. From a number of 

 introduced species which grow in this region either as winter 

 annuals or as summer annuals, it might be inferred that any 

 rapidly-maturing, short-lived species having seeds which begin 

 growth either at low temperatures or at high ones and sufficiently 

 tolerant to the temperature during its period of growth might 

 be able to grow successfully in competition with our native 

 winter annual or summer annual species. In the further study 

 of these groups of plants the writer has this suggestion under 

 consideration. 



Having in mind the relation already noted between climatic 

 factors and our vegetation forms, it will be interesting to observe 

 the more successful vegetation forms among our cultivated 

 species. The flowering plants which do best at the lower 

 altitudes in southern Arizona during the winter and spring 

 months, so far as the writer has observed, are the common 

 pansy, candytuft, migonette, sweet alyssum, sweet peas, nas- 

 turtiums, scarlet flax, lupines, California poppy, Mexican even- 

 ing primrose, Nemophila, Nigella, opium and Shirley poppies 

 (Papaver sps.), annual larkspur (Delphinium ajacis), corn 

 flower (Centaurea cyanus), pot marigold (Calendula officinalis), 

 Linaria canadensis, Gaillardia pulchella, Platystemon califor- 

 nicus and Linum Lewisii,* the four latter occurring native over 

 southern Arizona as winter annuals. From custom the above 

 flowers have come to be sown in September and October at 

 which time their seeds begin growth; this growth is continued 

 until the colder winter weather, when, unless protected, they 

 pass through a more or less inactive period, some even developing 

 rosettes. With the return of the more favorable temperatures 

 of late winter active growth sets in again, after which they 

 blossom commonly until the beginning of June, when, even 

 though abundantly watered, they usually die as a result of the 

 hot, dry weather. They are almost without exception annual 

 species with us, and for the most part short-lived; in their 

 manner of growth they behave quite similar to the winter 

 annual species already noted. Among cultivated vegetables 



At our higher altitudes this species grows as a perennial during the summer months. 



