CLASSIFICATION OF PLANTS. 125 
as the scarlet runner, they come above ground when the young plant 
begins to grow ; in others, as the bean and pea, they remain underground. 
Cotyledons exhibit a great variety of forms, and are sometimes wrinkled, 
curved, folded, or twisted. Those of the potato are spiral. Cotyledons 
are sometimes split into parts, as in firs, so that a seed appears to have many 
cotyledons instead of two. Plants of which the seeds have two cotyledons 
are called Dicotyledonous, from the Greek dis, twice. Those of which the 
seeds have only one cotyledon are called Monocotyledonous, from the Greek 
monos, one. These terms designate two great classes of plants, which include 
all the higher kinds of plants; but the lower kinds have an embryo 
entirely cellular, which is called a spore,! and gives forth a new root or 
stem from any part of its surface, according to the circumstances in 
which it is placed, and not necessarily from a particular point, as 
is the case in’ a monocotyledonous or dicotyledonous seed. Such plants 
are designated Acotyledonous, from the Greek privative a. There is 
a remarkable correspondence between the structure of the seed and the 
structure of the stem, so that exogenous plants are dicotyledonous, endogenous 
plants are monocotyledonous, and acrogenous plants are acotyledonous, and 
into these three great classes all plants are divided. 
Classification of Plants. 
About 120,000 species of plants are known, some of them so minute, 
that they are mere objects of microscopic examination, while others 
are of vast magnitude. The great object of botanists in their systems 
of classification is to exhibit the scheme of nature itself; and this is 
in a great measure accomplished by thedivision of plants into the three 
great classes just mentioned. Plants which have much agreement in 
structure and characters, are grouped together as forming one order, and 
those in which the resemblance is still closer as forming one genus. 
Acrogenous Plants.—Of the lowest class of plants, known as Acrogenous, 
Acotyledonous, or Cryptogamous,? which have no flower, the lowest of all 
are found in the order Alg@ [ Latin, ‘ sea-weeds or water-weeds’]. These 
abound both in the sea and in fresh water in all parts of the world. Some 
sea-weeds attain a great size, exceeding in lencth the tallest forest trees. 
They have no root, but are merely attached by their base to rocks, 
imbibing all their nourishment from the water in which they float. Some 
of the Alge are used for food, as dulse and carrageen or Irish moss. 
Some of the larger sea-weeds are valuable as yielding kelp, which is 
obtained by burning them, Kelp, the ashes of sea-weed, contains a large 
1 From Greek sporos, seed. 
2So called from their fructification being concealed, from Greek kryptos, concealed, and 
gamos, marriage. 
