38 -Plant-life of the Oxford District 



arvense and E. Telmateia of the swamp, also intrusive in pastures, arable 

 fields and hedges). 



Not only may species of the samegemts show range of type from shrub 

 to herbaceous perennial, or from perennial to biennial or annual, according to 

 features of somatic organization, or time-factors of the reproductive period, 

 but in the same genus representative or complementary species commonly 

 diverge to different biological lines or stations, as xerophytes v. subaquatics, 

 summer-flowering v. spring-flowering, climbers v. non-climbers, etc. ; since, 

 as already noted, natural relationship is expressed in the more fundamental 

 features of racial mechanism traced in the details of the reproductive pro- 

 cesses, and seen most readily in the floral structures rather than in the 

 vegetative shoot-system. 



To sum up, the general case of the progression of the Herbaceous 

 Xerophyte is sufficiently familiar, as its response to the necessities of a 

 short season cuts its working history down to the stage of the biennial, 

 annual, or even * ephemeral ', or leads to a multiseasonal and monocarpic 

 habit. The vegetative shoot-construction has lost its woody texture, the 

 shoot-system is specially adapted to perennate over the dry season in 

 rosette or rhizome-form. In the limit, the subterranean rhizome, with its 

 special cases of corms, tubers, and bulbs, affords a familiar example of 

 getting beneath the surface of the protective soil and stopping there ; while 

 in the short favourable part of the year the reproductive system may long 

 tend to repeat, as far as possible, in its inflorescence-axis, the original erect 

 branching habit of the arboreal prototype. Under these conditions, the 

 flowers may retain an organization as fully efficient, quite as ' primitive ', or 

 even more suggestively ' elementary ' and archaic than those of any forest- 

 tree (cf. Helleborus, Aquilegia, with Magnolia and Liriodendron). The 

 same mechanism which is effective during heat-perennation, is usually also 

 efficacious for cold and frost-perennation (especially once well below the 

 surface of the ground), and the perennation-problem is so far unified. The 

 active life of the plant, including more especially its reproductive cycle, is 

 thus condensed within the limit of the favourable growth-period, as deter- 

 mined more particularly by the possible water-supply. 



Given the sunlight and temperature of any part of the world in tropical 

 to temperate regions, the critical factor in determining the range of forest- 

 canopy is the question of adequate water-supply, whether at some time 

 too little, as expressed in low rainfall, in impervious soil, extreme evapora- 

 tion, rocky ground, etc., or too much, tending to swamp areas insufficiently 

 aerated, muddy bottoms with no holding ground, grading to deep and open 

 water of standing ponds, streams, lakes, and even rivers in which the current 

 introduces an additional factor. To the former case belong the wide range 

 of reduced herbaceous types conveniently classed as Xerophytes ; * to the 

 latter lies open an equally wide scope of adaptation to conditions of aquatic 

 environment, in which the essential water-problem no longer obtains, though 

 temperature and light in the end become limiting factors. 



Of all such reduced herbaceous flora, adapting their shoot-mechanisms 

 to these conditions, the rhizomatous type is perhaps the most interesting ; 

 since with the prostrate habit, and loss of negative geotropism in the vege- 

 tative shoots, together with the retention of the possibility of intercalary 

 extension (or even with practically none at all), there remains a definite 

 mechanism for lateral transportation, which is beyond the power of the 

 normal ancestral tree. With this capacity for lateral extension and adven- 

 titious rooting, such types of vegetation can travel in the course of time 

 considerable distances, with the chance of finding conditions more suit- 



1 Schimper (1903), Plant Geography, p. 3. 



