228 - Multicellular Plants 

 FLOW E,R 



PISTIL 



STAMEN 

 PETAL 



SEPAL 



FLESH, FROM RECEPTACLE 



FRUIT 



Fig. 12-30. In pome fruits, such as 

 the apple, the bulk of the fleshy part 

 is derived from the receptable of the 

 flower. 



or two-seeded fruits but also the stony coat 

 directly surrounding the seed proper are de- 

 rived from the pericarp. The seeds of such 

 fruits are also distributed by animals, which 

 eat them. 



Fruits are also classified as to the number 

 of individual pistils that are originally rep- 

 resented in the flower. Accordingly: 



A. A simple fruit is one that has devel- 

 oped from a single pistil, and all the fruits 

 previously mentioned (except the burdock, 

 tomato, and maple) are simple fruits. 



B. An aggregate fruit is a cluster of rip- 

 ened ovule chambers, the pistils of which 

 were all borne in the same flower, for exam- 

 ple, raspberries and blackberries. 



C. A multiple fruit is a cluster of ripened 

 ovule chambers, the pistils of which were 

 borne in separate flowers, all having a com- 

 mon receptacle, for example, the pineapple. 



GROWTH AND DEVELOPMENT IN PLANTS 



In general, the development of plant struc- 

 tures is much simpler than that of animals. 

 Most parts of the plant are essentially solid 

 masses of cells, in contrast to the hollow or 

 tubular nature of many animal organs, Ani- 



mal development (see Chap. 15) involves a 

 complex series of foldings, invaginations, and 

 cell migrations; but plant development oc- 

 curs mainly at localized growing points, 

 where the cells multiply, grow, and differ- 

 entiate, forming the tissues of the adult 

 organs. 



In thalloid plant bodies, the growing points 

 are mainly localized at the apex of each fila- 

 ment, or lobe. At this point one usually finds 

 a single large cell, the apical cell, which di- 

 vides continually, producing all the cells of 

 the thallus. The apical cell remains more or 

 less permanently in an undifferentiated con- 

 dition, and retains an unlimited capacity for 

 multiplication and differentiation. But most 

 of the other cells become differentiated sooner 

 or later, forming the specialized tissues in 

 the different parts of the thallus. 



Among higher plants, which possess roots, 

 stems, and leaves, cell division is largely re- 

 stricted to growing points, which are local- 

 ized masses of embryonic (meristem) tissue. 

 The meristem cells remain in an undiffer- 

 entiated state indefinitely, and retain their 

 capacity for unlimited multiplication and 

 differentiation. In the root, the growing point 

 is located near the tip of each branch (Fig. 



