74 
Then a paper. "Some (.hiecnsland Mangrove Barks and Other Tanning 
.Materials." by .1. ('. Briinnicli and K. Smith (Qiimtduml .\<ifn;il1tmil .Itmrwil xxvii, 
86, 1911) may bo referred to. It contains useful information in regard to the Mangroves 
of 'Northern Queensland, and incidentally to the bark-eollecting. There are notes as 
to the area covered, and to the relative abundance of the stilt-roots. 
Mr. Charles Hedley refers to Avicennia of/id imli* in. his most interesting Presi- 
dential address before the Royal Society of New South Wales, Journal, Vol. xlix, 45, 
46 (1<> !.->). and published two plates, 1 and 2, the latter, and Mr. Baker's xlviii, showing 
the breathing-roots, or Cobbler's Pegs, excellently. 
A few months later, op. fit., p. 257, Mr. R. T. Baker published an exhaustive 
and copiously illustrated monograph on Avicennia ojficin<rli*. I am only incidentally 
referring to this species at this moment, and content myself with drawing attention to 
those portions of the paper referring to breathing-roots (pneumatophores). At p. 263 
Mr. Baker makes the interesting observation that the shade of the tree is necessary for 
the welfare of the breathing-roots underneath. At p. 204, &c., he deals with the 
anatomy of these roots. 
Knees in Cypress trees. Aerating roots. Origin and meaning of " knees " in 
Conifers (Cypresses, &c.). 

The effect of the quantity of water in the soil or of growing in a water medium is very marked on 
most plants, but has not. been of great importance in inducing variations in cultivated plants. Serious 
lack of water (a condition which is found in deserts and sandy regions) has given rise to various devices 
by plants to prevent loss of water by evaporation from the leaves, water storage reservoirs in the tissue, 
specialised glands to absorb dew, &c. Desert trees and shrubs are commonly stunted, gnarly-stemmed 
plants, with large root systems. The fact that these characters almost invariably disappear (frequently 
in the first generation) when the plants are grown where there is an abundance of water and food, shows 
that they were assumed because of a lack of these materials. 
The bald cypress (Taxodium distichum) furnishes an interesting illustratio'n of the effect of excess 
of water. The cypress, as is well known, grows usually at the present time in swamps and very wet places. 
Geological records, however, show that centuries ago, previous to the Glacial epoch, the cypress tree grew 
in the present Arctic region, associated with oaks, maples, &c. As it was forced southward by the gradual 
change in climate, competition with other trees evidently resulted in its present habit of growing only in 
swamps. Plants growing on dry land secure the necessary oxygen needed m root growth from the air, 
which is always present in the soil. Plants growing in the water or on very wet soil, however, frequently 
find it difficult to secure sufficient oxygen, and this has led to the development of devices to facilitate the 
iteration of the tissue. Cypress trees growing in water form numerous protuberances on the roots known 
as "cypress knees." wh'ev ; >ove the water into (.he air. Ky growing numerous seedlings of the 
cypress under va: iitions, L)r. Wilson has shown that these roots arc invariably formed by plants 
growing in water, and are neve- I'ormi d ivln-ti the -plants are grown on fairly dry soil which contains 
sufficient, air. lie r, :!ud. lint 1 hese pec.ulii'.r crguns enable the loots of the tree to secure 
tin- oxygen, and are developed us ;:, direct result i,f the hiibii assumed by the cypress of growing 
in swamps. It is an interesting fact thet this habit of forming knees, which was acquired centuries ago, 
has IP. t become heredit;i: y. being ; the first generation if the tree is grown on dry soil. In 
swamps and on lake i .. hich place < are now its natural home, the buld cypress forms a ragged, 
spreading growth, with large limbs and sparse foliage, and. is very different from the common type of 
closely related pine trees. This also is the result of a lack of oxygen and food, as before stated. When 
the tree is grown on dry soil, as it frequently is in parks, where it secures abundant air and nutrition, it 
