PLANT ORGANS AND ORGANISMS 1 97 



is found the embryo sac or megaspore containing protoplasm and a 

 nucleus, (See Fig. 97 A). 



The coats do not completely envelop the nucellus, but an opening 

 at the apex, called the foramen or micropyle admits the pollen tube. 

 The vascular plexus near the point where the coats are attached to 

 each other and to the nucellus is called the chalaza. The hilum 

 marks the point where the funiculus is joined to the ovule, and if 

 attached to the ovule through a part of its length, the adherent 

 portion is called the raphe. The shape of the ovule may be ortho- 

 tropous, or straight; campylotropous, bent or curved; amphitropous, 

 partly inverted; and anatropous, inverted. The last two forms are 

 most common. A campylotropous ovule is one whose body is bent 

 so that the hilum and micropyle are approximated. 



The Placenta. The placenta is the nutritive tissue connecting 

 the ovules with the wall of the ovary. The various types of pla- 

 centa arrangement (placentation) are gouped according to their 

 relative complexity as follows: (i) Basilar, (2) Sutural, (3) Parietal, 

 (4) Central, (5) Free Central. 



Basilar placentation is well illustrated in the Polygonacea (Smart 

 Weed, Rhubarb, etc.) in Piper and Juglans. Here, at the apex of the 

 axis and in the center of the ovarian base, arises a single ovule from 

 a small area of placental tissue. 



Sutural placentation is seen in the Leguminosce (Pea, Bean, etc.). 

 Here each carpel has prolonged along its fused edges two cord-like 

 placental twigs, from which start the funiculi or ovule stalks. 



Parietal placentation is seen in Gloxinia, Gesneria, Papaver, etc. 

 Here we find two or more carpels joined and placental tissue running 

 up along edges of the fused carpels bearing the ovules. 



Central or axile placentation is seen in Campanulacece (Lobelia), 

 where the two, three, or more carpels have folded inward until they 

 meet in the center and in the process have carried the originally 

 parietal placenta with them. This then may form a central swelling 

 bearing the ovules over the surface. 



Free Central placentation occurs perfectly in the Primulacece, 

 Plantaginacece and a few other families. In this the carpels simply 

 cover over or roof in a central placental pillar around which the 

 ovules are scattered. 



