476 



BOTANY 



PART II 



tliallium-cell is to be recognised, the remainder of the small prothallium 

 representing one or more antheridia. The female protliallium, which 

 in Salvinia still becomes green and emerges from the macrospore, in 

 Selaginclla and Isocies has lost the power of independent nutrition. 

 The prothallium begins its development while still within the macro- 

 sporangium of the parent plant, and the macrospore, after being set 

 free, only opens in order to allow of the access of the spermatozoids to 

 the archegonia. 



Thus within the series of Vascular Cryptogams a reduction 

 of the sexual generation can easily be traced ; this reduction is 

 carried further in those Phanerogams which stand nearest to the 

 Cryptogams (-). 



The MACROSPORE, which in the Phanerogams is termed the 

 EMBRYO-SAC, remains enclosed in the macrosporangium or ovule. 



A 



S 



c 



Fin. 422. — A, Atropous ; B, aiiatrnpons ; C. caini)ylotropow.s ovules. 

 (I)iagraniniatic and iiiagnifipd.) 



The latter consists of the NUCELLUS, from the base of which (the 

 CHALAZA) one or two integuments arise ; these grow up as tubular 

 investments of the nucellus and only leave a small passage, the 

 MICROPYLE, leading to the tip of the latter. The ovule is attached 

 to the MACRO-SPOROPHYLL or CARPEL by a stalk or funiculus, which 

 is often very short. The region to which one or more ovules are 

 attached is called the PLACENTA. If the nucellus forms the direct 

 continuation of the funiculus the ovule is termed straight or ATROPOUS. 

 More frequently the funicixlus is sharply curved just below the 

 chalaza, so that the ovule is bent back alongside its stalk (anatropous 

 ovule). The line of junction of the funiculus with the outer integu- 

 ment is still recognisable in the ripe seed, and is termed the raphe. 

 Lastly the ovule itself may be curved, in which case it is spoken of as 

 CAMPY LOTROPOUS. The three types are diagraramatically represented 

 in Fig. 422 A-C. 



As a rule only one embryo-sac is contained in an ovule. In the 

 same way as the four macrospores originate by the tetrad division in 

 the macrosporangium of Selaginclla, in the macrosporangium (ovule) of 



