472 BOTANY PAKT n 



undergoes still further reduction. In the germination of the micro- 

 spores only a single, vegetative prothallium-cell is to be recognised, 

 the remainder of the small prothallium representing one or more 

 antheridia. The female prothallium, which in Salvinia still becomes 

 green and emerges from the macrospore, in Selaginella and Isoetes has 

 lost the power of independent nutrition. The prothallium begins 

 its development while still within the macrosporangium 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 ( 2 ). 



The MACROSPORE, which in the Phanerogams has for long been 

 termed the EMBRYO-SAC, remains enclosed in the MACROSPORANGIUM 



C 



Km. 432. A, Atropous ; B, anatropous ; '', eampylotropous ovules. 

 (Diagrammatic and magnified.) 



or OVULE. 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 funiculus 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 

 CAMPYLOTROPOUS. The three types are diagrammatically represented 

 in Fig. 432 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 



