OTMNOSPERM^. 393 



parasitic or saprophytic. They range from minute plants 

 one or two centimetres in height, and living but a few days 

 or weeks, to enormous trees, which continue to grow for 

 many hundred years, and which attain a diameter of ten, 

 and a height of one hundred metres. 



5O4. The Phanerogams are separable into two classes, 

 as follows :* 



Class I. Gymnospermae (the Archespermce of Strasbur- 

 ger). The ovules are not enclosed in an ovary. The en- 

 dosperm arises before fertilization, and forms rudimentary 

 archegonia ("corpuscula"), in which the germ-cells origi- 

 nate. The contents of the pollen grains divide before the 

 growth of the pollen tube, forming a rudimentary pro- 

 thallium, much as in Selaginellce and Isoetece. 



Class n. Angiospermse (the Metaspermce of Strasburger). 

 The ovules are enclosed in an ovary. The endosperm is 

 formed after fertilization. The contents of the pollen grain 

 remain undivided before and during the growth of the pollen 

 tube. 



Sub-Class Monocotyledones. The first leaves produced by the 

 embryo (the cotyledons) are alternate ; the endosperm is usually large 

 and the embryo small. 



Sub-Class Dicotyledones. The first leaves of the embryo form a 

 whorl of two (i.e., they are opposite); the endosperm is very often 

 rudimentary or entirely wanting, and the embryo is generally large. 



II. CLASS GYMNOSPERMAE. 



505. The plants of this class have solid stems, which 

 bear in most cases small, simple, narrow leaves having a 

 parallel venation. The xylem portions of the fibre-vascular 

 bundles of the stem are closely compacted into a single dense 

 woody cylinder, which is surrounded by a looser mass of 

 tissues, the so-called bark, composed of the united phloem 

 portions of the bundles. The woody cylinder increases its 



* This is essentially Sachs' arrangement, in his " Lehrbuch," 4te 

 Auf. The terms Archespermae (from the Greek apxri, beginning, and 

 therefore properly Archespermse, instead of Archisperrnse) and Meta- 

 spermae (from perd, after or later) are those proposed by Strasburger : 

 " Die Coniferen und die Gnetaceen," 1872, p. 239. 



