FOURTH GROUP. 



THE SEED-PLANTS. 



(SPERMAPHYTES OR PHANEROGAMS.) 



THE characteristic feature in the Phanerogams is, that the alternation of 

 generations in them is concealed in the formation of the seed, which in the primary 

 state at least consists of three parts, the seed-coat, the endosperm x , and the embryo 

 which is the product of fertilisation in the oosphere. A seed is formed when the ripe 

 macrospore is not released from the macrosporangium, but remains enclosed in it and 

 there produces prothallium, archegonium, and embryo. The seed therefore is 

 simply a macrosporangium modified in a peculiar manner, which separates from 

 the asexual generation and encloses the macrospore with the prothallium and 

 embryo 2 . 



We have seen the SEXUAL GENERATION, the PROTHALLIUM, which is 

 formed directly from the spore, losing more and more the character of an independent 

 plant in the Vascular Cryptogams. In homosporous Ferns, Ophioglosseae and 

 Equisetaceae, it vegetates often for a long time independently of the spore ; in hetero- 

 sporous Ferns and Lycopodineae it is formed inside the spore; in the former the 

 female prothallium is thrust forth from the cavity of the macrospore, but continues to 

 be connected with it, but in the Isoeteae it fills the interior of the macrospore as a 

 mass of tissue, which only bursts the coat of the spore to make the archegonia 

 accessible to the spermatozoids. In the Cycadeae and Coniferae this retrogressive 

 metamorphosis goes a step further ; the prothallium, here termed the endosperm, always 

 remains enclosed in the macrospore, the embryo-sac, where, as in the Vascular Cryp- 

 togams, it produces archegonia, formerly known by the quite superfluous name of 



1 The ripe seeds of many Dicotyledons have no endosperm, because it has been consumed and 

 displaced by the rapidly growing embryo before the seed is matured ; this takes place in other 

 seeds after maturity in germination, that is, during the unfolding of the embryo. More rarely the 

 formation of endosperm is from the first rudimentary. 



2 Macrosporangia and microsporangia, macrospores and microspores have a special terminology 

 in seed-plants, which is now in many cases obsolete and which will be considered further on ; but 

 the structures thus named are throughout the same as the sporangia and spores in the Vascular 

 Cryptogams. 



