71 6 ECOLOGY 



sary for their optimum development. Thus an obvious advantage of 

 these plants is that all their energy and material are consumed in the 

 development of organs directly concerned with nutrition and repro- 

 duction. A great disadvantage of the annual habit arises from the 

 necessity that the entire race start anew from seed each year ; thus the 

 shortness of the season excludes annuals from alpine and arctic regions, 

 and maximum foliage display is imp3ssible for such forms in any 

 region. Probably their chief disadvantage is that ultimately they are 

 excluded from most habitats by perennials, owing to the increasing pre- 

 emption of ground by the perennating organs of the latter. Annuals 

 reach their culmination in open situations, as in deserts and in culti- 

 vated fields, and along shores and roadsides. 



Ground perennials. Plants whose perennating organs are rhizomes, 

 bulbs, corms, tubers, runners, rosettes, or multicipital stems may be 

 termed ground perennials; such plants have a high measure of pro- 

 tection with a minimum utilization of structural material. The deeper 

 the organ, the more complete is its protection from cold or transpira- 

 tion, while the shallower the organ, the less is the amount of material 

 consumed in reaching the surface at the inception of the growing 

 season. Rhizome and runner plants surpass all other land plants from 

 the standpoint of vegetative propagation, and bulbs, tubers, and corms 

 are especially advantageous by reason of their abundant food supply, 

 which facilitates the rapid development of aerial organs. Rosettes and 

 multicipital stems have similar advantages, if the roots contain abun- 

 dant food (as in the dandelion and the dock), though such habits are 

 poorly suited for vegetative reproduction. Because the new shoots 

 each year arise from the soil level or below, ground perennials, like 

 annuals, are unable to display a maximum amount of foliage. How- 

 ever, rhizome perennials have about the most advantageous of plant 

 habits, since they exhibit the combination of an enduring horizontal 

 stem with a periodic erect stem; such a combination results in maxi- 

 mum protection with a minimum utilization of structual material, in 

 maximum reproduction, and in adequate, though minor, foliage dis- 

 play. Plants with bulbs, tubers, or corms are suited especially for 

 districts with short vegetative periods, as the Mediterranean region, 

 where the winter is too cold and the summer too dry for optimum 

 growth activity, the most favorable seasons being periods of short 

 duration in the spring and autumn ; in these plants much of the 

 food utilized in a season's growth is accumulated the year previous, 



