G68 
POLLINATION OP FLOWERS 
ding, also was developed. Many plants pro¬ 
duce spores in countless numbers: the com¬ 
mon puffball has been estimated to contain 
three trillions, the spores issuing under 
pressure like puffs of smoke. They are also 
very abundant in mosses and ferns. 
THE AGE OP SEAWEEDS. 
The first period in the histoi'y of plant 
life was the age of seaweeds, and during 
this long interval of time there were no 
land plants. In the warm paleozoic seas 
more than one hundred million years ago 
seaweeds were more abundant than they 
are today. The lower or seedless plants 
are of great interest because they show the 
various stages by which stems, leaves, 
roots, and sexual reproduction have been 
gradually developed from microscopic, one- 
celled plants. Step by step in the Algae 
and ferns the way was prepared for the 
evolution of the seed-plants. Without 
these lower groups the early history of 
plants would be far more obscure than it 
is at present. Animal life with its power 
of motion found the conditions in the 
ocean suitable for a varied and advanced 
development; and, responding to all the 
complex action of the sea, it branched and 
perfected itself before the evolution of 
land animals. All the great divisions of 
the animal kingdom, including the lower 
vertebrates or fishes, are represented. But 
plants could make little progress in the 
ocean and did not advance beyond the sea¬ 
weeds. The development of the plant world 
has taken place almost wholly upon the 
land. The advance of plants from the 
ocean to the land is justly regarded as the 
most important event in the history of 
vegetation. 
THE FIRST LAND PLANTS. 
The first land plants were liverworts, 
mosses, and ferns. The simplest liverworts 
which are little more than green mats, with 
lobed margins, growing on damp ground, 
probably resemble very closely the first 
land plants. Prom such seaweed-like 
plants, which had become only partially 
adapted to living on the land, are descend¬ 
ed by different lines the mosses and ferns. 
The life cycle, or history of each moss and 
fern, consists of two different phases or 
generations, the earlier portion being a 
sex plant, the later, a spore plant. Both 
kinds of reproduction are apparently al¬ 
most of equal advantage to the plant. 
The sex plant in the ferns is a small 
flat brown mat or thallus, growing on the 
ground. It has neither stems nor leaves 
and closely resembles a seaweed, which is 
a strong reason for believing that all of 
the mosses and ferns have come from sea¬ 
weed-like ancestors. The sex plant pro¬ 
duces both eggs and sperms in separate 
organs which, so long as it is dry, remain 
closed; but, as soon as they are covered 
with water by rain or overflow, they open 
and the sperm swims by whip-like hairs to 
the ovary and fertilizes the egg. Thus 
fertilization in the mosses and ferns is the 
same as in the seaweeds, and in all three 
groups water is the means by which it is 
effected. The sex plant is indeed an aquatic 
plant, while the spore plant is a land plant 
—mosses and ferns are thus amphibians. 
They have never become wholly independ¬ 
ent of their aquatic origin. After fertili¬ 
zation the egg grows into an entirely dif¬ 
ferent plant, the leafy fern or spore plant. 
It is the spore plant that is commonly 
known and admired for its beautiful 
fronds. On the back of the fern leaves 
there grow round spore cases or sporangia, 
in which are produced in great numbers 
yellow spores of the same size, which are 
widely distributed by the wind. Each of 
these spores under favorable conditions 
grows into a sex-plant. Since sex-plant 
and spore plant thus continuously succeed 
each other, this life cycle is called alterna¬ 
tion of generations. 
HETEROSPORY. 
The spores produced by most fernworts 
are of equal size, and grow into sex-plants 
which produce both sperms and eggs. But 
a few of the higher ferns and club-mosses 
have produced spores of two different 
kinds, small spores and large spores—het- 
erospory. Both kinds are widely distrib¬ 
uted by the wind. The small spores grow 
into male plants which produce only 
sperms; the large spores grow into female 
plants which produce only eggs. Cross- 
fertilization is a frequent result of this ar¬ 
rangement; but a vast number of spores 
are wasted since the two kinds of spores 
are often so widely dispersed and are so 
