172 OVULE OF [CH. XIII 



Ovule. 



The structure of the ovule may be studied in the 

 marsh-marigold (Caltha palustris), a plant which has 

 already been utilised for the study of the anther. Caltha 

 belongs to the Ranunculaceae, and like the buttercup it 

 has a group of free carpels in the centre of the flower. 

 Each carpel resembles a miniature pea-pod, and contains 

 several ovules borne on the united edges of the carpellary 

 leaf. The ovule or immature seed is attached to the 

 carpel by a stalk known as the funicle, by which food is 

 supplied to it from the mother plant. The scar left at 

 the point of attachment of the funicle to the seed has 

 already been described, under the name of the hilum 

 in Chapter II. The ovule consists of a mass of simple 

 cellular tissue, the nucellus, n in fig. 80, within which is 

 contained the egg-cell. The nucellus is covered by an 

 integument, i, which on the left side of the nucellus (away 

 from the funicle) is seen to be made up of two layers. 

 The integuments do not completely shut in the nucellus, 

 a narrow gap is left at m leading from the cavity of the 

 ovary down to the nucellus. This passage is the micropyle 

 which persists in the adult seed as a hole in the seed-coats 

 (see the drawing of the seed of Vicia faba, fig. 4, p. 17). 

 In the ovule its function is to admit the pollen-tube by 

 which fertilisation is effected. If a line is drawn along 

 the funicle as far as the base of the ovule, and then 

 through the longer axis of the ovule to the micropyle, the 

 result will be a curved line f| like the letter U reversed. 

 When this is the case, so that the micropyle is close to 

 the point of origin of the funicle, the ovule is described as 



