FERTILIZATION. 45 



single cotyledon forms a minute sheath (a), enclosing 

 the plumule (b), the radicle (c) being here but little 

 developed at first, the greater part of the grain con- 

 sisting of the albumen (d) ; the grain should be soft- 

 ened in water before examination. In the germi- 

 nated grain the cotyledon appears as a pale sheath, 

 surrounding the convolute green leaves of the plu- 

 mule ; which may be best seen in a transverse section 

 (PI. I. fig. 48). 



Fertilization. A few words must now be said 

 regarding the formation of seeds, and the action of 

 the pollen-tubes in the process of fertilization. 



In the earliest stages of growth, the young seeds, 

 or ovules as they are called, appear as little buds, 

 arising from the inner wall of the ovary ; and the 

 part from which they arise is called the placenta 

 (placenta, a cake). In chickweed (PI. I. fig. 41 c), 

 the placenta forms a central column ; and when the 

 ovules are a little older, they are found to have sepa- 

 rated somewhat from the placenta, but retaining a 

 connexion by means of a little cord or stalk, termed 

 the funic'ulus (funis, a cord) . The ovules may be 

 readily found in the ovary or young pod of a wall- 

 flower, the placentas forming four lines, running lon- 

 gitudinally down the interior of the pod. 



In this early condition the ovule consists of a mass 

 of cellular tissue ; and as new formations are soon 

 added to it, it is termed in this state the nucleus. 

 Around the nucleus are then formed two coats, an 

 outer, called the primine, and an inner, termed the se- 

 cun'dine. These coats or membranes are open at one 

 end, so as to leave a passage down to the apex of the 

 nucleus; the opening is called the foramen. These 

 structures are well seen in the ovule of the wallflower 

 (PI. I. fig. 54), the foramen in the figure being indi- 

 cated by a * ; it will be noticed also that the funi- 

 culus runs down one side of the ovule, so as to ter- 

 minate at the bottom or base of the nucleus. In 



