SEED-PLANTS. 



39 



in stems thai are increasing rapidly in thickness and forming wood soon gives rise to 

 a perfect ring by bridging over the primary medullary rays, and then constantly pro- 

 duces new layers of phloem on the outside and new xylem on the inside. In the 

 primary roots also and stronger lateral roots of Gymnosperms and Dicotyledons the 

 subsequent formation of a closed cambium-ring causes an increase in thickness, which, 

 like that of the stem, is not a characteristic of the Cryptogams, and frequently leads 

 to the formation of strong and persistent root-systems, often functionally replaced in 

 the Monocotyledons by rhizomes, tubers, and bulbs. Finally this long-continued growth 

 in thickness is connected with active and copious formation of cork, which usually 

 passes into the production of bark, a process foreign both to Vascular Cryptogams and 

 Monocotyledons. But these also are subjects which will be better discussed in detail 

 when we are describing the characters of the several divisions of the Phanerogams. 



CLASSIFICATION OF SEED-PLANTS OR PHANEROGAMS. The 



Phanerogams are distinguished from the Vascular Cryptogams (Pteridophytes) by 

 the formation of the seed, which is the product of the macrosporangium or ovule. 

 The ovule produces the embryo-sac in the nucellus which is its most essential part, 

 and in the embryo-sac are formed the endosperm and the oosphere ; the oosphere 

 is fertilised by the contents of the pollen-tube, an outgrowth from the pollen- 

 grain, and after fertilisation a pro-embryo is developed which differentiates into 

 suspensor and embryo. The phanerogamous plant with stem, leaves, roots and 

 hairs answers to the spore-producing generation (sporophore or sporophyte) of the 

 Vascular Cryptogams ; the embryo-sac is the macrospore, the pollen-grain the 

 microspore ; the endosperm, at least in the Gymnosperms, is the female prothallium 

 (oophore or oophyte), and the seed unites in itself at least for a time the two 

 generations, the prothallium or endosperm and the young plant of the second 

 generation, the embryo. 



I. PHANEROGAMS WITHOUT AN OVARY (GYMNOSPERMAE). 



The ovules before fertilisation are not enclosed in an ovary formed by the 

 cohesion of carpels ; the prothallium or endosperm is produced before fertilisation 

 and forms archegonia ; the pollen-grains undergo divisions of their contents before 

 the formation of the pollen-tube, such as take place in the microspores of the 

 Selaginelleae. 



Gymnospermae. The formation of leaves in the embryo begins with a whorl 

 of two or several members. 



A. CYCADEAE. Branching of the stem rare or altogether suppressed ; leaves 

 large, branched. 



B. CONIFERAE. Axillary branching copious, but not from all leaf-axils ; leaves 

 small, not branched. 



C. GNETACEAE. Mode of growth very various ; flowers in many respects like 

 those of Angiosperms. 



II. PHANEROGAMS WITH OVARIES (ANGIOSPERMAE). 



The ovules are produced inside an ovary formed of cohering carpels or of 

 one carpel with coherent margins, and having at its summit the stigma on which the 

 pollen-grains germinate. The endosperm is formed after fertilisation at the same 



