624 



WEED FLORA OF IOWA 



Seed. — The seed is the matured ovule. It contains the young 

 plant or embryo which is the essential part of the seed since it is 

 through the later development of this young plant that new in- 

 dividuals are produced. Accompanying this maturing of the ovule, 

 various other changes take place which give distinguishing features 

 to different seeds. Frequently the endosperm grows so extensively 

 as to absorb and replace the cells of the nucellus and thus comes 

 to occupy all the space within the coats of the integuments, as in 

 the morning-glory, onion, etc. The embryo may remain compara- 

 tively small as in the morning-glory or onion, or it may in turn 

 absorb and replace all the cells of the endosperm and so come to 

 occupy the space within the integuments, as in the bean and clover. 

 Sometimes some of the nucellus and endosperm remain. The in- 

 teguments also undergo various changes during the formation of 

 the seed, often becoming hard or papery or provided with hairs, 

 hooks or spines, cr becoming smooth or pitted. 



Fig. 491 I 



Fig. 491 II 



. Fig. 492 



Fig. 491. I- Immature pistils of Geranium. II. Mature pistil and carpels 



with their long styles are separating from the elongated axis. 



(After Thome.) 



Fig. 492. Burry fruit of Jamestown or Jimson Weed (Datura) showing method 



of dehiscing. 

 (After Thome.) 



Fruit. — A fruit is a ripened ovary alone or a ripened ovary plus 

 closely related parts such as calyx, involucre, and receptacle. In 

 the beggar-ticks (Bidens) the ovary becomes tough and invests the 



