REPRODUCTION AND DISPERSAL 



90s 



through the enlargement of the ovary with or without the modification 

 of other organs, is called the fruit, and obviously its chief role is the 

 protection of the developing seeds. Most young fruits contain sufficient 

 chlorophyll to make them conspicuously green, and they doubtless manu- 

 facture much of the food utilized in their development. Sometimes, as 

 in the elm, green fruits are prominent before the leaves appear, or, as 

 in Cakile, after the leaves have gone; in such cases the food-making 

 role would seem particularly significant. At maturation the fruit by 

 reason of its edibility and showiness often is a means of attracting seed- 

 dispersing animals, and in other ways it is connected with the process of 

 dissemination. 



General characteristics of seeds. — Morphology. — The seed, which is 

 an enlarged and matured ovule, is one of the most complex organs found 

 in plants. The ovule, which is a sporophytic organ, serves as a domi- 

 cile for the female gametophyte, and afterwards for the new sporophyte 

 that develops from the fusion of the sperm and the egg. This structure, 

 which thus is made up of elements from three generations, grows to a 

 certain size, varying with the species, whereupon the integument de- 

 velops into the seed coat or testa, and the growth of the sporophyte within 

 is checked. Henceforth the structure may be called a seed rather than 

 an ovule, so that a seed may be defined as a young sporophyte in a state 

 of arrested development, enclosed within a modified ovule integument, 

 the testa ; in addition, the gametophytic generation may or may not be 

 represented by the endosperin (but see p. 270). 



Developmental features. — The ovules commonly arise on the margins 

 of the carpels, to which they are attached by a stalk, ihe funiculus. The 

 central mass of tissue, 



the nucellus, usually is y'f'^ ^ z^'T"^ // 1\ '205 

 enclosed by one or two 

 integuments arising from 

 the basal region (chalaza) 

 just above the funiculus; 

 the integuments do not 

 close tightly about the 

 nucellus, but leave a 

 slender canal, the mi- 

 cropyle, through which 

 the pollen tube usually makes its way 

 funiculus (orthotropous) 



1204 



Figs. 1204, 1205. — 1204, cross and longitudinal 

 sections of a seed of Canna, showing the seed coat or 

 testa ((). 'li^ perisperm (e), and the embryo (p) ; 1205, 

 longitudinal and cross sections of a bean (Phaseolus), 

 showing the seed coat or testa (0, the cotyledons {c). 

 and the plumule (p). 



The ovules may be erect on the 

 partially pendent (campylotropous), or more 



