INTRODUCTION. 



497 



rays so as to form a perfect ring (the cambium-ring), and then produces regularly new 

 layers of phloem on the outside and of xylem on the inside. In the primary roots and 

 the stouter lateral roots of Gymnosperms and Dicotyledons an increase of thickness 

 also takes place by the subsequent formation of a closed cambium-ring, which, like that 

 of the stem, is not found in Cryptogams, and commonly leads to the formation of strong 

 persistent root-systems, which are more often replaced physiologically in Monocoty- 

 ledons by rhizomes, tubers, and bulbs. With the persistent increase in thickness is 

 connected, finally, the active and extensive production of cork, a process foreign both 

 to Cryptogams and to Monocotyledons. It will be more convenient, however, to defer 

 the special discussion of these points also until we are treating of the characteristics of 

 the separate classes. 



SYSTEMATIC REVIEW OF PHANEROGAMS. 



The distinguishing characteristic of Phanerogams, as contrasted with Cryptogams, 

 lies in the formation of the Seed. This organ is developed from the ovule, which, in 

 its essential part the nucellus, produces the Embryo-sac, and in this the Endosperm and 

 the Oosphere. The latter is fertilised by the Pollen-tube, an outgrowth of the Pollen- 

 grain, and, after, produces the Embryo borne on a Suspensor. The phanerogamic 

 plant which is differentiated into Stem, Leaves, Roots, and Hairs, corresponds to the 

 spore-forming (asexual) (Sporophore) generation of Vascular Cryptogams ; the Embryo- 

 sac to the Macrospore ; the Pollen-grain to the Microspore ; the Endosperm is equivalent 

 to the female Prothallium ; and the Seed unites in itself, at least for a time, the two 

 generations, the Prothallium (Endosperm), together with the young plant of the second 

 generation, the Embryo. 



Flowering Plants may be classified as follows : — 



I. Phanerogams without an Ovary. 



The ovules are not enclosed before fertilisation in a structure (the Ovary) resulting 

 from a cohesion of carpellary leaves. The endosperm arises before fertilisation, and 

 forms archegonia {i.e. * corpuscula '), in which the oospheres originate. The contents 

 of the pollen-grains are divided before the formation of the pollen-tube, corresponding 

 to divisions taking place in the microspores of Selag'mella. 



I. Gymnosperms. The first leaves produced from the embryo are arranged 

 in whorls of two or more. 



A. CycadecB. Branching of the stem very rare, or entirely suppressed; 



leaves large, branched. 



B. ConifercB. Axillary branching copious, but not from all the leaf-axils ; 



leaves small, not branched. 



C. Gnetacece. Mode of growth very various ; flowers similar in many 



respects to those of Angiosperms. 



II. Phanerogams with an Ovary. 



The ovules are produced in the interior of a structure (the Ovary) formed by the 

 cohesion of carpellary leaves (often only of one carpel, the margins of which have 

 become coherent), bearing at its summit the stigma upon which the pollen-grains 

 germinate. The endosperm is formed after fertilisation at the same time as the 

 embryo, both remaining rudimentary in some cases. A division of the contents of the 

 pollen-grain is indicated. The branching is almost always axillary and from the axils of 

 all the foliage-leaves. 



Kk 



