STEMS 715 



that are classed as biennials develop such shoots and become triennials 

 or even quadrennials (as in Artemisia canadensis, Arabis lyrata, and 

 some mutants of Oenothera Lamarckiana, figs. 716, 717). Plants that 

 are annuals in cold climates may be perennials in the tropics {e.g. the 

 castor bean), and it is possible to cultivate some annuals or biennials 

 as perennials by exposing them to favorable conditions during periods 

 of severity (as in the pansy). Many annuals and biennials die rela- 

 tively early in the vegetative season; for example, the staminate plants 

 of the hemp {Cannabis sativa) die in late summer, while the pistillate 

 plants with the developing seeds still are green and in full vigor. 

 Indeed, most annuals and biennials that fruit early in the season die 

 soon thereafter. The causes of such phenomena are unknown. 



Aquatic herbs. — In the water the protective habits of many plants 

 are comparable to those of the land, especially in such as have 

 rhizomes, but unattached water plants form a class by themselves. 

 Such floating plants, which include numerous large and small algae 

 and some ferns and seed plants, are among the best protected of 

 plants. The winter buds or shoots sink in autumn and rise in spring, 

 thus requiring no such utilization of energy and material as in the deep 

 placement of rhizomes and in the subsequent emergence of erect shoots. 

 The chief disadvantages inhering in aquatic habitats arise from the 

 instability of the water which makes impossible the growth of tall aerial 

 organs and from its high refrangibility, which permits only moderate 

 synthetic activity in submersed organs. 



The compensatory relations of plant habits. — General remarks. — 

 In the preceding paragraphs, reference has been made to the advantages 

 and disadvantages of the various classes of plant habits, and it may be 

 desirable, partly by way of summary, to contrast them further. In 

 general, the principle of compensation is illustrated, disadvantages being 

 offset by corresponding advantages. Erect and branching stems, whose 

 habit well suits them for optimum display, are poorly suited for vegeta- 

 tive reproduction, and the construction of their protective tissues re- 

 quires a great consumption of energy and material. On the other 

 hand, horizontal ground stems, which are well suited for vegetative 

 reproduction and which are protected with a minimum consumption of 

 material, are unsuited for optimum foliage display. 



Annuals. — Annuals as a rule are without conspicuous protective 

 structures; this is not a disadvantage, because the non-existence of 

 these forms during periods of severity makes protective tissues unneces- 



