SPERMATOPHVTA 



383 



evolution from the sporangium-bearing leaf of the ferns, which is often 

 interpreted as a modified branch system (see page 305). 



The Ovule. In the development of an ovule, at first a small rounded 

 protuberance appears (Fig. 327). This is the nucellus, or megasporan- 

 gium proper. At its base an integument then arises as a ring of tissue, the 

 nucellus meanwhile increasing in prominence. Later, if a second integu- 

 ment is to be formed, it arises outside the first one. The integument or 

 integuments grow out beyond the nucellus, leaving a narrow passageway, 



Fig. 327. Successive stages in the development of an anatropous ovule, the last repre- 

 senting a section through a mature ovule. (After Gray.) 



A B c 



Fig. 328. Directions of ovules: A, orthotropous; B, campylotropous; C, anatropous. 

 (After Coulter.) 



the micropyle. In the Archichlamydeae and Monocotyledoneae two 

 integuments are generally present, but in nearly all the Metachlamydeae 

 there is a single massive one. As a rule, the ovule is borne on a short 

 stalk, the funiculus, the part of the ovary to which it is attached being the 

 placenta. The basal portion of the ovule is called the chalaza. 



When mature, ovules may be erect {orthotropous), curved {campylotro- 

 pous), or inverted {anatropous) (Fig. 328). There are also intermediate 

 conditions. The first represents the most primitive condition and is 

 characteristic of most cauline ovules. It is found among the Urticaceae, 

 Polygonaceae, Xyridaceae, and a few other relatively primitive families 

 of Archichlamydeae and Monocotyledoneae. The second type is also 

 uncommon, being found among the Chenopodiaceae, Caryophyllaceae, 

 Cruciferae, and Gramineae. The third condition is most advanced and 

 most common. Anatropous ovules are found in many of the Archi- 

 chlamydeae and Monocotyledoneae, and almost exclusively in the 



