PISTILS \M> TIII.IK PARTS. 39 



viscid or sticky so as to catch the pollen grains. Sometimes the 

 stigma is borne directly on the ovary but more often it is a1 the top 

 or on the side of a slender stalk called the style, winch is an elonga- 

 tion of the upper part of the ovary, in corn the styles are very 

 long and form the so-called "silk." 



The ovary of a simple pistil when removed and cut crosswise is 

 seen to have hut a single cell or cavity to contain the ovules, while 

 that of a compound pistil may have a number of cells, their par- 

 tition walls being formed by the carpels which compose the ovary. 

 The number of these carpels can usually be told by the number of 

 styles or stigmas present. In the corn cockle there is one com- 

 pound pistil with 5 styles, a stigma being attached along the in- 

 side of each, so that 5 carpels were united to form the pistil. 

 When cut crosswise the ovary is seen to be only 1-celled, the car- 

 pels not having formed complete partitions or walls though traces 

 of such walls are seen at the base. The ovules are many and are 

 joined to a central column which extends from the bottom to near 

 the top of the ovary. The ovules of different weeds are arranged 

 in the cells in different ways, sometimes being joined to one or 

 more seams (placenta) on the sides of the cell and sometimes at- 

 tached to a central column as in the cockle. If single they may 

 be attached at base to the bottom of the cell. 



The number of cells in the ovary is used as a very important 

 character in distinguishing the families of plants and can usually 

 be readily determined by carefully removing the ovary, cutting it 

 crosswise with a sharp knife and then gently squeezing the ovules 

 from one of the halves. By looking closely with a lens the number 

 of small cavities can then be easily counted. (Fig. 12, d, e, /.) It 

 is also important to know whether the ovary is in any way united 

 to the calyx or not. In the corn cockle the two are wholly sepa- 

 rate, the ovary being above the calyx, and when so placed it is 

 said to be superior or free. In the evening primrose and many 

 other weeds the calyx is partly or wholly united to the ovary and 

 the latter is then said to be in ft rior. 



Within each ovule is a little embryo sac containing a minute 

 egg or germ. When a pollen grain falls on the sticky stigma it 

 develops or sends out a very slender tube containing numerous 

 microscopic sperm or male cells. This finds its way down through 

 the tube of the style and entering the ovule through a minute open- 

 ing empties the sperm cells into the embryo sac. One of the sperm 

 cells unites with the egg and the fertilization of the latter re- 

 sults. From the fertilized egg the young or embryo plant is pro- 



