712 ECOLOGY 



considerable energy and material are utilized in the development of 

 protective tissues and in complete leaf renewal each year; also decidu- 

 ous trees are inferior to sclerophylls in the amount of synthetic activity 

 on favorable days during the leafless period and in leaf protection on 

 unfavorable days during the period of leafage. 



Evergreen herbs. — Many herbs, especially in the tropical rain forest, 

 are, like the trees, evergreen, presenting the same aspect at all seasons. 

 There are some such plants, even in periodic climates, as the scouring 

 rush (Equisetum hyemale, figs. 1054, 1055) and the prickly pear 

 (Opuntia, figs. 1040-1042), which are leafless evergreen herbs well- 

 fitted to withstand exposure. Also there is a large class of low herbs 

 with sclerophyllous evergreen leaves, as Linnaea, Mitchella, Cornus 

 canadensis, and the wintergreens (Pyrola, GaiiUheria, Chimaphila). 

 Another class of herbaceous evergreens includes forms which, at least 

 in winter, are without aerial stems (as Hepatica, Mitella, Geiim, and 

 Polystichum) ; in this group are a number of species with relatively 

 mesophytic leaves which readily survive the winter if transpiration is 

 prevented by coverings of leaves or snow. Some herbaceous ever- 

 greens are more xerophytic, occurring in exposed situations (as Ar- 

 temisia, Taraxacum, Lepidium, and Oenothera, fig. 1036); when such 

 plants are not protected by coverings of leaves or snow, many of the 

 outer leaves die, but the younger leaves within are uninjured. Most 

 of the xerophytic evergreen herbs and shrubs of cold climates form 

 ground rosettes or have prostrate stems, and in many cases the aerial 

 organs are arranged in cushions (fig. 1060) ; in all such cases closeness 

 to the ground or to other organs reduces transpiration and lessens the 

 detrimental effect of sudden changes of temperature. Among the 

 evergreen herbs should be classed most of the lichens, liverworts, and 

 mosses, their small size often insuring sufficient winter protection by 

 snow and leaves; furthermore, most lichens and many mosses are quite 

 unharmed by months of exposure to transpiration without absorption. 

 Such plants as the melon cacti (Echinocactus, fig. 1063) may be re- 

 garded as evergreen herbs which are the extreme antithesis of tropical 

 evergreen trees, having, on account of their spherical shape, the least 

 possible transpiring surface in proportion to volume, and therefore 

 illustrating the culmination of protective form among aerial organs. 



Deciduous herbs. — In the great mass of herbs, particularly in 

 periodic climates, aerial stems as well as leaves die at the inception of 

 the unfavorable season. The death of the stem is not, as in the case 



