EVERGREEN CONIFERAE 311 



tissue : ' The gymnospermic " xerophily " is not the result of even inherited 

 adaptations to dry conditions, is not in fact an ecological adaptation in 

 the usual sense, but is a result of their histological structure (which is 

 incapable of allowing a rapid flow of water through the wood), their 

 wood consisting entirely of tracheids, which are usually pierced by 

 " bordered pits"; the diameter of the tracheids is less than that of the 

 vessels of the flowering plants, and the whole structure of the wood is 

 simpler and more uniform.' The Gymnosperms are a very ancient and 

 primitive group ' in which the woody conducting-system has not reached 

 the state of specialization and efficiency which was afterwards attained 

 by Angiosperms.' Miss Stopes comes to the following conclusion : * It 

 appears then that the xerophytic characters of the Coniferales in very 

 many cases are not adaptations to xerophytic conditions in their own 

 lives, nor are they " inherited " from the remote past as vestigial characters 

 no longer in touch with present-day necessities, but are the result of 

 physiological limitations of the type of wood in this ancient and in- 

 completely evolved group. In other words, their 'xerophytism' is not 

 oecological, but phylogenetic.' 



It may seem to be beyond doubt that the xerophytism of Coniferae, 

 as well as the almost complete prevalence of the evergreen habit, are 

 phylogenetic characters derived from the archetype, and that the xero- 

 phytic structure of the foliage is correlated with the primitive structure 

 of the conducting-tissue. But Miss Stopes is incorrect in assuming 

 that Coniferae nowadays do not for the most part grow under dry con- 

 ditions of life. As already indicated, all evergreen trees compelled to 

 experience a cold winter must necessarily be capable of withstanding 

 a season that is rigidly dry in a physiological sense ; and when they grow 

 upon raw humus or peat the soil is relatively physiologically dry, even 

 though it drip with water. 



The primitive structure of the wood of the Coniferae may accentuate 

 their xerophytic external form, and may be the phylogenetic cause of 

 this ; yet it cannot be regarded as wholly answerable for the xerophytism 

 which now prevails, since coniferous forest at present mainly occurs on 

 physically or physiologically dry soil. 



Inasmuch as so many of the coniferous trees, Picea excelsa, for instance, 

 are shade-enduring trees, the vegetation clothing the ground is very 

 scanty because the forest is very shady, in virtue of the numerous foliaged 

 shoots, whose leaves allow the passage of no light and remain attached 

 throughout the year. The deep shade therefore endures all the year. 

 Consequently, under various conifers, Picea excelsa and Abies pectinata 

 for instance, raw humus readily arises. 



The humble plants clothing the soil in northern coniferous forest are all 

 perennials, which vary in construction of shoot and other characters. 



Dwarf-shrubs and undershrubs are numerous, and include species of 

 Vaccinium, Ledum, Calluna, Empetrum, and Juniperus ; and to these we 

 may append species of Pirola. The majority of these shrubs, like many 

 of the herbs, are evergreen. 



Creeping rhizomes, or roots that emit buds, are possessed by not a few 

 species, including those of Pyrola, Monotropa, Maianthemum, Goodyera 

 repens, Oxalis Acetosella, Trientalis europaea, Vaccinium Vitis-idaea and 

 V. Myrtillus, Pteris aquilina, and Aspidium Dryopteris : this may possibly 



