498 PHANEROGAMS. 
2. Monocotyledons. The first leaves produced from the embryo are alternate; 
endosperm usually large ; embryo small. 
3. Dicotyledons. The first leaves of the embryo form a whorl of tvi^o (or are 
opposite) ; endosperm very often rudimentary, often entirely absorbed by 
the embryo before the ripening of the seeds. 
CLASS X. 
GYMNOSPERMS 
This class embraces, in the orders Cycadeae, Conifeise, and Gnetace^, plants 
of strikingly different habit, but evidently closely allied in their morphological 
structure, in the peculiarities of the mode of formation of their tissue, and espe- 
cially in their sexual reproduction. On these grounds they take up an interme- 
diate position between Vascular Cryptogams and Angiosperms, while they approach 
Dicotyledons among the latter, especially in their anatomical structure. 
The Pollen-grams suggest a homology with the microspores of Selaginella^ 
their contents undergoing before pollination one or more divisions into cells which 
resemble a very rudimentary male prothallium. One of these cells (the largest) grows 
into the pollen-tube when the pollen-grain has reached the nucellus of the ovule. 
The pollen-sacs are always outgrowths from the under side of structures unquestion- 
ably foliar (staminal leaves), and bear a striking resemblance in many cases to the 
sporangia of some Vascular Cryptogams. They are produced either in larger or 
smaller numbers or in pairs on a staminal leaf, without cohering in their growth. 
The Ovule, which is almost always orthotropous, and usually provided with only 
one integument, either appears to be the metamorphosed end of the floral axis itself, 
or it originates laterally beneath its apex (or is apparently axillary), or it grows from 
the upper surface or margins of the carpels. These never cohere so as to form a true 
ovary before fertilisation, although during the ripening of the seeds they often in- 
crease considerably in size, close together, and conceal the seeds, usually separating 
again when they are mature in order to allow them to fall out ; the cases are, however, 
not rare in which the seeds remain quite naked from first to last. The embryo-sac 
is formed beneath the apex of the ovule, which consists of small-celled tissue and 
remains enclosed until fertilisation by a thick layer of the tissue of the nucellus. 
Sometimes the formation of several embryo-sacs commences in one nucellus, but 
only one of them attains its full development. The Endosperm arises by free 
cell-formation long before fertilisation in the embryo-sac, which is distinguished by 
its firm wall ; but the cells soon become combined into a tissue and increase by 
division. Within this mass of tissue, corresponding to the endogenous prothallium 
of Selaginella, arise the Archegonia (or Corpuscula^) in larger or smaller numbers. 
^ [The central cells of the archegonia of Gymnosperms were discovered by Robert Brown in 
1834. He called them corpuscula or embryoniferous areolae (Miscellaneous Botanical Works, 
