Introduction. Plants are the most abundant group of organisms of the planet, that produce organic material from the inorganic material and, therefore, allow the existence of animals and fungi, which are organisms that need organic matter to feed themselves. Among the plants we can distinguish two groups:
In this chapter we study the mosses and ferns. They were the first organisms that achevied to live out of water. Before them, about 400 million years ago, there was only life in the water. The ferns, as they already have vessels, internal conductors, they may have meters high and constitute large forests. The subtraction of which originated in the current coal deposits, and provided the leaves, which were eaten by large herbivorous dinosaurs. In the jungles of Central and South American ferns can still be seen over 15 meters high. It is very interesting to know what they are and how they reproduce , since they are organisms that have survived for so long, approximately 200 times more than we hominids.
Activities to do. Read the explanations on the General Characteristics and Classification of Plants, and do the Multiple Choice Test 13.1 and Relate drawings and names 13.1. Then read the text on the ferns and do the multiple choice Test 13.2, the Relate drawings and names 13.2, and the Crossword 13.
Based on these concepts, classification of plants is as follows:
2 . Mosses. They are photosynthetic organisms with a waterproof epidermal tissue and some conducting cells, but without actually forming a genuine conducting tissue.
Thanks to possessing
a waterproof skin that prevents desiccation, they were the first
photosynthetic organisms that colonized the continents. Because of lack
of a conducting tissue, water has to move from cell to cell.
This inefficient mechanism causes that they do not exceed more than a
few centimeters high. They have structures similar to roots, stems and
leaves but which actually are false roots, false stems and
false leaves that have no internal conducting vessels.
They appeared from
an evolution of some specie of algae, which lived in the rolling waves.
Most of them live in very humid areas such as forest soils. Others are
aquatic. Some species are able to live in very dry areas, stay dry endure
several years and move to the active life when it rains again. This allowed
them to live in inhospitable envirinment as on rocks, walls, roofs, etc..
They present asexual
reproduction through fragmentation and sexual reproduction with
alternation of generations. This consists in the fact that the moss
has two forms, one with filament form with many false green leaves and
which is very abundant, and another smooth filament shaped brown that
is not abundant. Green forms generate male gametes and female
gametes and hence are called gametophytes. If there is water,
male gametes swim to female gametes to fertilize and thus generate a zygotic
cell which begins to multiply and generates a brown form. This, as
an adult, produces sexual spores (spores with half the genetic
information) and, therefore, this form is called the sporophyte.
Subsequently, these spores land in the ground, germinate and each gives
rise to a green form. Green form cells have half of chromosomes (condensed
DNA molecules) that brown form cells and, therefore, it is said that in
sexual reproduction of mosses, thay alternate two forms, one of which
has double chromosomes as the other.
to a conductive tissue they can distribute water effectively throughout
the plant absorbing itfrom the ground. Because of this, they can reach
heights of 1 to 2 meters in temperate steady countries and up to 16
meters in tropical forests. As they have conductive tissue they give
real roots, real leaves and real stems. Same as
mosses, appeared from evolution of one green algae species.
mosses, they present asexual reproduction from rhizome
which does not stop to grow, and sexual reproduction alternation
of generations. Unlike mosses, the greatest and most abundant
form is produced by the sexual spores, that is the sporophyte (which
in our country have a size from 25cm to 2 m in height) and form small
and little abundant is which form gametes, ie gametophytes (having
a size of about 5 or 6 millimeters).
The sporophyte has a very large leaves called fronds, which come out of a horizontal underground stem called rhizome, from which arise numerous roots. The fronds are usually very divided and when young are wound on themselves in the form of cane. On the back there are a yellowish spots, sores, within which are the sporangia which are the organs where sexual spores are formed. They are called that because they are spores that have only half of genetic information (half the chromosomes) in their stem cells. From each one of them, a gametophyte may be formed, that is a very small film called protalo. On its lower side, which is in contact with water, bumps appear with male gametes within and larger other protrusions with one single female gamete inside. If there is enough water in the ground, male gametes go swimming and fertilize the female and this is when generate a zygote cell that begins to multiply and generates a form that will end up giving a new sporophyte.