596 METASPERMAE OF THE MINNESOTA VALLEY. 



habit of growth or habit of nutrition in the plants in question. 

 Under these retarding influences the gradual development of 

 monocarpic into polycarpic forms, of herbaceous into shrubby 

 and of shrubby into arborescent types may be brought in evi- 

 dence. As the distribution of the herb is more rapid than that 

 of the shrub and the distribution of the shrub more rapid than 

 that of the tree, any influences that induce the emergence of 

 shrubby or arboreal characters may be deemed distinctly re- 

 tarding in their general effect. That such a difference of 

 mobility actually exists is derived from the testimony both of 

 experience and of a priori reasoning. The adventive plants 

 and the escaped plants in any region are always in large part 

 herbaceous, because it is more easy for plants of small size and 

 rapid maturation to gain a foothold than for plants of large 

 size and slow maturation. But in the internal competition for 

 light — the important energising force of plants — the emer- 

 gence of the shrubby or arboreal character may be expected 

 and precisely as it becomes more prominent— unless other 

 modifications arise to maintain the general equilibrium — 

 will the rate of distribution decrease. In general we see that 

 the higher forms of archichlamydeous trees such as the linden, 

 the maple, the walnut, are heavier seeded than the lower forms 

 such as the willow or the poplar. The increased size of the 

 seed is necessary to provide for the increased difficulties that 

 surround the establishment of the seedling. So thus it is 

 evident that the development of the arboreal type exerts a 

 retarding influence upon distribution. 



Fluctuation in tensions. Fluctuation's in equatorial pres- 

 sure may arise in several ways. Beside the general accelera- 

 tion due to the increased extension of the central groups of 

 species and formations and the general retardation due to the 

 causes mentioned, there will arise fluctuations which may 

 originate in widely diverse conditions. These conditions may 

 be topographical, climatological, geological — in the widest 

 sense — or biological. The erosive action of streams, by re- 

 ducing the general altitude of a tract of country, brings about 

 alterations in the rates of plant movement over such a tract. 

 And by the reduction in altitude, changes in annual rainfall, 

 annual temperature, mean direction of winds, and in maxima 

 or minima of each of thes3, are brought about. Or again secu- 

 lar changes in the general level, due to orogenic movements in 

 the crust of the earth, may induce greater or less fluctuations 

 in the rate of movement of the line of tension, as they are 



