SECT. II 



PHANEROGAMIA 



515 



tip enters the mieropyle of an ovule and so reaches the apex of the nucellus. Of 

 late years a number of exceptions to this usual course have become known. Treub 

 first showed in Casuarina that the pollen-tube entered the ovule by way of the 

 chalaza, and thus reached the peculiar sporogenous tissue, which in this case 

 develops a number of embryo-sacs. CHALAZOGAMY, as this mode of fertilisation is 

 termed in contrast to POUOGAMY, has no special systematic significance, since it is 

 found to occur in numerous and unrelated plants. As NAWASCHIN has shown in 



Kit;. -I'.'li. Ovaiv ( Polygonvm I'^ 

 vulus (luring t'tTtilisation. fa, Stalk- 

 like base ol n\:iry : /, fnilictllu.s ; chit, 

 chalaxa ; *u, niicellns ; mi, mirropyle ; 

 ii. iniiei 1 . i' . outer integument; e, 

 embryo-sac ; ek, nucleus of embryo- 

 KOC ; c), ege;-appiiratus ; mi, antipodal 

 ('11s; ;;. style; n. stijjma; />. l>ollen- 

 Krains ; /, pollen-tubes, (x 48.) 



Fifi. 500. Ovule of Ulmun i'ihinculatti. <, Kmbryo- 

 sac ; in, iiiici-n|iylf ; //, diala/a : t, |Hicket-like .s))aci' 

 between the inte^iiiiients. Tin 1 jKdlen-tulx', ]ix, pene- 

 trates dii-fctly tliiDux'U the two integuments and 

 reaches the apex of the nucellus. ( After NAW \scnis.) 



the case of Ulmus, the pollen-tube in all these plants attempts to avoid growing 

 across hollow cavities, possibly because it needs to be continually nourished at the 

 expense of the surrounding tissue. A fuller understanding of chalazogamy and 

 related phenomena is approached if they are regarded from this biological point of 

 view (Figs. 499-501). 



When the pollen-tube, containing the t\vo geiirmtivr o-lls. h;is readied the 

 embryo-sac, its contents escape and pass by way of one of the synergidae to the 

 ovum ; the corresponding synergida then dies. One of the two generative nuclei 

 fuses \\ith the nucleus of the ovum, which then becomes surrounded by a cellulose 

 wall. The second generative nucleus passes the ovum and unites with the large 



