1470 



A TEXTBOOK OF THEORETICAL BOTANY 





■^' \*^"WA 



example, they are also known in Ranunculus ficaria and in Corydalis cava, 

 both autotrophic species, and in Thismia americana, which unlike other 

 members of the Burmanniaceae is not saprophytic but which shares the 

 family character of an embryo arrested at a very early stage. On the other 



hand the embryos of the parasitic Loran- 

 thaceae and Santalaceae, though in some 

 respects anomalous, do not appear to be 

 arrested. 



The term suspensor is applied to the 

 structures produced from the " basal " 

 cell formed at the first division of the 

 zygote, in other words the cell nearest to 

 the micropyle. It may consist only of 

 one much enlarged cell, as in embryos of 

 the Caryophyllad Type, but more usually 

 it consists of a row of cells, or even of a 

 multicellular mass, forming a sort of 

 pedestal to the embryo proper, and serv- 

 ing to attach it to the micropylar end of 

 the embryo sac (Fig. 1349). It is a very 

 variable organ, for it serves not only as an 

 attachment for the embryo, a requirement 

 that does not seem to be always necessary, 

 but also, in many instances, for the con- 

 veyance of nutriment to the embryo. In 

 many species the cells of the suspensor 

 elongate in a manner which recalls the 

 lengthy suspensors common among 

 Gymnosperms, pushing the pro-embryo 

 downwards into the midst of the endo- 

 sperm. This is characteristic of the long 

 narrow embryo sacs of the Sympetalae. 

 The embryo in such cases is presumably 

 capable of absorbing nourishment from 

 the endosperm directly through its own surface and the function of the 

 suspensor is purely mechanical. Such suspensors may either shrivel up 

 when their function has been accomplished, or they may remain, as in 

 Trapa natans, even to the maturity of the seed, being compressed by the 

 growth of the embryo into a solid cap over the tip of the young radicle. 

 Some of the longest known suspensors are found in members of the Loran- 

 thaceae, such as Dendrophthoe, where the embryo sacs penetrate far up the 

 style. After fertilization the suspensors push the pro-embryos right down 

 into the ovary, there to complete their development. 



Elongated suspensors may also play a part in embryo nutrition. This 

 is conspicuously so in the Papilionaceae (Fig. 1350), where the suspensor 

 cells multiply and enlarge so that each cell may become larger than the 



Fig. 1349. — Capsella biirsa-pastoris. 

 Section of ovule with de\elop- 

 ing embryo, showing suspensor 

 formed of a column of cells 

 with a much enlarged basal cell. 



