ORGANS OF EEPEODUCTION. 



321 



Fig. in. 



have an ascending ovnle, while those of Poteriu'in, Sanguisorha, 

 &e., have it suspended, and in Potentilla, both ascending and 

 suspended ovules are found. In the Eanunculacese also we find 

 the ovule varying in like manner as regards its position. 



We will now consider the position of the ovules Avhen their 

 number is more than one. Thus when the ovarj^ or cell has 

 two ovules (bioimlate), these may be either placed side by side 

 at the same level, and have the same direction, as in Nuttalia, 

 when they are said to be collateral ; or they may be placed at 

 different heights, and then either follow the 

 same direction, when they are superposed, or 

 one ovule may be ascending, and the other 

 suspended, as in JEscuIks {fig. 717). The po- 

 sition of the ovules also, in those cases where 

 they are in definite numbers, is usually con- 

 stant and regular, and similar terms are em- 

 ployed; but when the number of ovules in 

 the ovary or cell is indefinite, the relations are 

 less constant, and depend in a great measure 

 upon the shape of the cell, and the size of 

 the placentas. Thus in the long ovaries of many 

 of the Leguminosse {fig. 651) and Cruciferse 

 {fig. 601), the ovules are superposed, and by pig, 

 not crowding each other they will all be turned 

 in the same direction ; while, on the contrary, 

 if the ovules are numerous, and developed in a 

 small space, they will necessarily crowd each 

 other, and acquire irregular forms and varying 

 positions, according to the direction of the pres- 

 sure. In describing these varying positions 

 the same terms are used, as those referred to 

 when speaking of the relations of the solitary ovule. These 

 terms are also applied in the same sense to the relations of the 

 seed in the pericarp. 



Formation and Structure of the Ovuxe, — The ovule 

 appears at first as a little roundish celhdar projection on the 

 placenta; this gradually enlarges and acquires ultimately a 

 mor^ or less ovate or somewhat conical form ; this body is 

 termed the micleus {fig. 718). It is at first perfectly uniform 

 in texture and appearance, presenting no cavity as distinct from 

 those of the ordinary parenchymatous cells of which it is com- 

 posed, and having no integuments. As development proceeds a 

 cavity is formed at or near the apex of the nucleus (/^. 719, c), in 

 which the embryo or future plant is developed ; hence this cavity 

 is called the emhryo-sac or sac of the amnios. In rare cases, as in 

 the Mistletoe, two or three embryo-sacs are formed, This sac is 

 either formed by a simple hollowing out of the nucleus, and the 



Vertical 

 section of a cell 

 of tlie ovary of a 

 species of ^Sscu- 

 /«^ containing two 

 ovules, o, 0, one of 

 which is ascend- 

 ing and the other 

 suspended; m,'m, 

 niicropylc ; s. base 

 of the '^tyle. rrom 

 Jussieu. 



