Seaweeds and Leaf-green 
cellulose 1s burnt up. Carbonic acid gas, steam (water), 
as well as light and heat reappear. 
But for such a fire a supply of oxygen is essential. 
So when the sugar is formed in nature, which is the 
reverse process, we find radiant energy, carbonic acid 
and water being used up, but we also find that surplus 
oxygen is given off. 
All green plants possess this power of forming new 
organic material out of inorganic carbonic acid and 
water, but there are certain interesting differences 
between them. 
The first and lowliest group of plants, the seaweeds 
or alge, are not all green. 
It has long been noticed that on an ordinary rocky, 
gently sloping seashore where seaweeds flourish, there 
are green, brown, and red kinds. 
Close to the level of high tide one finds bright, grass- 
green enteromorphas and ulvas. The territory of the 
slimy wracks and tangles or kelp, which are mostly of 
an olive or reddish brown, lies between high and low 
water-mark, 
At about the lowest level of the tide and below it toa 
considerable depth, one finds neither green nor brown 
algz, but only those graceful and delicate red, pink, or 
purplish-red forms which are the ones usually selected 
for preserving on blotting-paper. At a certain depth 
one finds no living algz of any kind. 
No one could help speculating upon these colour 
changes, and to-day we are almost in sight of a complete 
explanation. 
The deep blue colour of the ocean is connected with 
the fact that other colours in the sunlight, and especially 
those red-yellow rays, which are the most important for 
plant-life on land, are absorbed by comparatively shallow 
depths of sea-water. This point was ingeniously tested 
31 
