GVMNOSPERMS. 433 



embryo, both remaining for a time rudimentary. The contents of the pollen-grain 

 do not divide. The branching is almost always axillary and from the axils of all the 

 foliage-leaves ; it is rarely extra-axi41ary. 



Glass XII. Monocotyledons. The first leaves produced from the embryo are 

 alternate ; endosperm usually large ; embryo small. 



Class XIII. 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 XI. 



G Y M N O S P E R M S. 



This class embraces, in the orders Cycadeos, Conifera^, and Gnetacese, plants 

 of strikingly diffcronl habil, but evidently closely allied in their morphological 

 structure. In the peculiarities of the mode of formation of their tissue, and espe- 

 cially of iheir sexual reproduction, they occupy an intermediate position between 

 Vascular Cryptogams and Angiosperms, while they approach Dicotyledons among 

 the latter especially in their anatomical structure. 



The PoUen-graim 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 grows into the 

 pollen-tube when the pollen-grain has reached the nucleus of the ovule. The pollen- 

 sacs are always outgrowths from the under side of structures unquestionably 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, appears to be either the metamorphosed end of the floral axis itself, 

 or 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 nucleus. 

 Sometimes the formation of several embryo-sacs commences in one nucleus, but 



F f 



