272 



REPRODUCTION 



chalaza and micropyle are all in the same straight line, as at 

 r, Fig. 98 : such ovules are described as oi-thotropous . 



When the ovule during its development becomes inverted as 

 at 2, Fig. 98, the micropyle lies close to the funicle : this form 

 is met with in the majority of common flowering plants, and is 

 spoken of as an anatropoiis 

 ovule. Among cruciferous 

 plants, and also among the 

 Caryophyllacese and Cheno- 

 podiaceae, the ovules are 

 more or less kidney-shaped, 

 the nucellus and integu- 

 ments being curved or bent : 

 ovules of this type are- 

 described as campylotropous . 



At an early period in the 

 development of the ovule a 

 specially large cell termed 

 the onbryo-sac makes its ap- 

 pearance in the tissue of the 

 nucellus at a point near the 

 micropyle of the ovule. 

 Within it a series of seven 

 new cells are developed 

 somewhat as follows. The 

 primary nucleus of the 

 embryo-sac first divides, and 

 the two halves then travel 

 to opposite ends of the cell. 

 Here each of these two new 

 nuclei divides into four, so that at this stage eight nuclei are 

 present, each of which has a certain portion of cytoplasm 

 associated with it. After this, one of the nuclei from the 

 chalazal end and one from the micropylar end travel back to 



3 



4 



Fig. 98 — I. External view of an orlhotro- 

 pous ovule. 2. The same of an anatropoiis 

 ovule. 3. Longitudinal section of i. 4. 

 Longitudinal section of 2. / Funicle ; 

 m micropyle ; h chalaza ; c coats of ovule ; 

 n nucellus ; e embryo-sac. 



