Subordinate and Herbaceous Flora 37 



ditions, repeated 2-3 times within the year, may suffice to produce the 

 seeds of a new generation ; an extreme case characteristic of several common 

 weeds. 



(2) The Biennial, or two-season plant, in which the short duration of 

 the annual working-season may be compensated by a perennation-phase 

 over the first winter, with postponement of the fruiting stage to the second 

 summer ; affording incidentally an admirable case for exploitation of stored 

 reserves at the end of the first season. 



(3) In more tropical regions, with no frost, but xerophytic endurance 

 of long-continued drought, such monocarpic plants may extend their work- 

 ing-period over many short vegetative seasons, and finally fruit out on an 

 intensive scale ; but no examples of this type occur in indigenous flora, so 

 that its occasional observation in garden-cultivation (cf. Agave) attracts 

 attention. 



Effective methods of herbaceous perennation, originally the response to 

 conditions of extreme desiccation over a dry season, may prove equally 

 efficacious against the extremes of Climate, as expressed in exposure to 

 cold winds at great elevation, or to the cold and wet of a dark Northern 

 winter. Hence in a North Temperate climate the herbaceous perennials 

 of a warmer region, with acquired effective perennation mechanism (especi- 

 ally in the extreme form of rhizome-extension J below the frost level, or by 

 the attainment of marked annual periodicity) may grow in a short hot 

 summer in a manner recalling the vegetation of even a tropical aspect. The 

 latter is the general experience of garden-cultivation in this country (cf. 

 Heliantkus annuus, Zca Mais, Cucurbita Pepo, Riciuns, Nicotiana) ; but 

 even in indigenous flora the last strays of essentially tropical types may 

 extend northward as woodland forms, the last representatives of their special 

 lines, commonly presenting other extreme phases of somatic or biological 

 organization. Cf. Cnscnta, holoparasite and annual; Calystegia, the most 

 sensitive indigenous stem-twiner, rhizomatous ; Bryonia, rhizomatous, and 

 the most perfect tendril-climber ; Taimts, rhizomatous, and a twining Mono- 

 cotyledon ; also the last residual Orchids (Platyanthera, moth-pollinated, 

 Listera, fly-pollinated) of epiphytic origin. 



An interesting case of the water-problem is seen in the herbaceous 

 Xerophytcs which vegetate in early spring to flower during the hottest 

 period of the year, often with extreme xeromorphic adaptations against loss 

 of available water, as spinous Thistles, Dipsacus, hairy and glandular Labi- 

 ates, succulents of walls and stony places (Sedum, Setnpervirum), as also 

 the plants of roadsides and waste de'bris, enduring more concentrated soil- 

 solutions (Chenopodium album, Polygonum avictilare, and the originally 

 halophytic Beets and Mangel). 



All degrees of complexity obtain, as what is commonly isolated as 

 apparently a special modification to one end, may more or less satisfactorily 

 solve several distinct problems ; so that plants cannot be accurately or 

 mechanically graded. One adaptation may play into others according to 

 edaphic changes, as well as climatic. A mechanism of perennation, originated 

 in response to extreme drought, may be equally effective against loss of 

 absorptive function in cold wet soil. Plants originating as special types of 

 dry stations with minimum water-supply, may endure swamp-conditions 

 which imply reduced root-activity. In this way xerophytes may apparently 

 grow as hygrophytes ; and the latter if they can invent a method of check- 

 ing excessive transpiration, may be found in xerophytic stations. (Cf. Iris of 

 sandy deserts and Iris Pseudacorus of the ditch-side ; Polygonum amphibium 

 flowering as an aquatic, and vegetating in dry sandy pastures ; Equisetum 



1 Cf. Hop, Rhubarb, Rumex Hydrolapathum, Bryonia, Asparagus, Calystegia. 



