i6 SEEDS : STRUCTURE AND GERMINATION 



root of the reversed seed in about twelve hours begins to turn downwards, 

 while the plumule more slowly bends in such a way as to assume the position 

 it had before it was reversed. The bottle should be placed in a dark box oi 

 cupboard to avoid the influence of light on the plant, and fresh air should be 

 blown into the bottle twice a day. 



8. Although seeds vary almost indefinitely in regard to size and 

 shape they are similar to the bean in so far as they all contain a 

 young plant packed away within the seed-coats. In this essential 

 feature all seeds agree with few exceptions, and it is on account of 

 the existence of a young plant within them that they are of use 

 in the raising of crops or plants. 



The manner, however, in which the embryo is arranged, and 

 the relative size and appearance of its various parts, differ con- 

 siderably in seeds; moreover, the growth during and after 

 germination is not the same in all cases. A few of the more 

 important and common variations in these respects must be 

 noticed. 



White Mtistard.— The seed of white mustard {Stnapis alba 

 L.) contains an embryo which like that of the bean consists of 

 a radicle, plumule and two cotyledons; the latter, which are 

 folded together, are relatively thinner than those of the bean and 

 deeply notched as in Fig. 5. The radicle is bent round and lies 

 in the fold of the cotyledons, between which is the very small, 

 almost invisible plumule. 



On germination the cotyledons, instead of remaining within 

 the seed-coat and below the ground as in the case of the broad 

 bean, escape from the enclosing coats altogether and grow up 

 out of the ground, enlarging at the same time, and becoming 

 green like ordinary leaves. They are the first ' smooth ' leaves 

 of the seedling mustard plant. 



After a short time the plumule grows up from between the 

 cotyledons and forms a stem upon which are gradually unfolded 

 the ordinary divided ' rough ' leaves. 



Ex. lO.^Soak white mustard seeds, and examine their structure, noting 

 especially the way in which the embryos are packed in them. Allow some to 



