104. THE AUSTRALIAN NATURALIST. 
Later the cotyledons fall. 
The presence of curious vertical aerating roots has been re- 
corded previously in this Journal. 
Note that the free swimming spermatozoids or generative cells 
of the pollen grain are the last trace of the algal ancestry in the 
higher plants. These motile sperms were discovered by a Jap- 
anese scientist. This discovery forms an interesting link in the 
chain connecting the higher and lower plants. 
DAWSONIA POLYTRICHOIDES, A HARDY MOSS. 
By Agnes A. BREWSTER. 
Mosses are descendants of certain algae, and are intere-ting 
because they show stages in tLe evolution of plants from an 
aqueous existence to that of land conditions. They develop stems 
in order to support chlorophyll-bearing organs or leaves. Then 
special areas must be set aside for the carrying of raw-food 
material up to the leaves, and for taking the elaborated foods 
made in the leaves, away to other parts: thus differentiation 
of tissue is produced. But the special structures thus formed 
are very simple compared with those of higher plants. 
Dawsonia is a sturdy moss which can be found growing on 
the roadside, or in.more sheltered conditions. It shows more 
complexity of structure than most mosses, which enables 10 to 
weather through moderate drought conditions. 
If a cross section (Fig. 1) of a stem be taken the following 
can be seen: (a) A central cellular area with thickened walls 
containing some very specially thickened and pitted-walled cells, 
called tracheal cells. This central area is called the hydrome, 
and.is surrounded by a ring of thicker cells called the hydrome 
sheath. The hydrome conducts the raw food material from the 
soil. 
(b) Surrounding: the hydrome is a zone of tissue which con- 
ducts the elaborated food made in the leaves, down to the other 
parts. It is called the leptome. 
Outside the leptome is the (c)cortex which free a protective 
covering, and whose cells may store food. In the cortex are 
primitive “veins” leading into the leaves; such ‘are called leaf 
traces (Fig. 1d). ; 
