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 two (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 Gnetaceae, 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-grains 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 foHar (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, 



