38 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



require eight or ten years to reach maturity. This is well exemplified 

 by Erythronium albidum, which each year produces one-leaved 

 flowerless shoots until the contractile roots have drawn the bulb down 

 to the proper level for reproduction. In nature, it is not usual for 

 trees to produce flowers under ten years, and in some soecies the 

 period of youth is doubled or trebled. 



The life history of the shoot is essentially a matter of the reciprocal 

 development of leaf and bud. Each leafy axis arises from a bud and 

 in its turn gives rise to new buds, typically in the axils of leaves, but 

 also at other places on stems, and even on roots. The further develop- 

 ment of a branching plant, especially a woody one, is determined by 

 the relation of terminal to axillary buds and the outcome of their 

 competition for food. At this point, development passes over into 

 the characteristic features of the life form. 



Movement. In spite of their stationary nature, flowering plants all 

 possess the power of movement in a restricted sense, for example, 

 growth and the circumnutation of stem and root tips. More pro- 

 nounced and less general are such tropisms as the turning toward 

 light, water, etc., the opening and closing of flowers, and the changes 

 in position displayed by flowers and fruits. Clamberers owe their 

 habit primarily to growth, supplemented by petioles, prickles, spines, 

 etc.; twiners ascend by means of a spiral movement; and climbers, by 

 virtue of tendrils, rootlets, or specialized petioles. Leaves exhibit a 

 variety of movements, from the active ones of sensitive plants and 

 flytraps to the slow but much more common ones resulting in the day 

 and night positions connected with the so-called sleep of plants. These 

 occur especially in compound leaves and hence are of frequent occur- 

 rence in the pea family. 



Propagation. This term is here employed in its botanical sense to 

 apply to asexual multiplication by natural rather than artificial 

 means. It involves something more than mere increase, inasmuch as 

 duration and migration are also connected with the process. Among 

 flowering plants, it occurs rarely with annuals, though the character- 

 istic tillering of grains and other annual grasses might be included 

 here. It is not a common feature of trees and shrubs, except when 

 the regenerative process has been set up through some accident, and 

 is then largely confined to angiosperms. While present in other eco- 

 logical groups, propagation attains its greatest expression in peren- 

 nial herbs. In fact, these owe their distinctive habit to this process, 

 and their life forms are determined by the manner in which this func- 

 tion is carried out. Process and form are so intimately and obviously 

 related that it is impossible to consider one without the other, but 



