1488 



A TEXTBOOK OF THEORETICAL BOTANY 



Palmaceae, or others which are richly supplied with integumental bundles, 

 are much to be desired. 



Such a developmental study has been made by Corner in the x\nnonaceae. 

 He showed that in this family the growth of the chalaza begins at an early 

 stage of ovular development, the original body of the ovule, with free 

 integuments, being carried away from the apex of the funicle and remaining 

 in a rudimentary condition round the micropyle, while the chalaza forms 

 the body of the ovule, its vascular bundle extending around the ovule in a 

 hoop. In Annonaceae this extension of the chalaza is confined to the median 

 plane, the integuments remaining free on the two flanks of the seed. A 



third, middle integument arises after fertiliza- 

 tion as a further development from the chalaza. 

 "How far these conditions may be paralleled 

 in other families is not known. 



Imprisoned within its testa the embryo 

 plant is an independent organization, cut oflF 

 not only from external influences, but also 

 from the parent plant (Fig. 1361). The forma- 

 tion of the corky covering of the hilum and the 

 suberized layers in the chalaza, which cover 

 the basal gaps in the integumental cuticles, 

 antedate the actual separation of the seed from 

 the placenta. Stress must be laid on the im- 

 portance, as a protective layer, of the inner 

 cuticle of the inner integument. The inter- 

 mediate and outer cuticles may indeed also 

 remain during ripening to contribute their 

 protection, but they often disappear, while the 

 inner cuticle almost invariably remains and fre- 

 quently increases in thickness. There are only 

 the rarest exceptions to this, Aescuhis being 

 the best known. This cuticle is highly im- 

 permeable to water and to gases and also to 

 dissolved substances. It is a complete barrier 

 to the entry of Bacteria or Fungi to the em- 

 bryo. This applies even in species where a 

 necessary mycorrhizal Fungus inhabits the 

 seed coat. It never penetrates to the resting 

 embryo, which only receives infection during 

 germination. The whole of the complex outer 

 coat may be rotted away or may be removed 

 by maceration in strong chromic-sulphuric acid mixtures, and yet the inner 

 cuticle will remain intact and the embryo unafl'ected. Such remarkable 

 impenetrability enables us to understand how so many small seeds can 

 be apparently sufficiently protected by the thinnest of cellular coverings 

 and, on the other hand, how some seeds can survive drastic chemical 



Fig. 1 36 1. — Urtico dioica. 

 Longitudinal section of 

 akene with the single 

 orthotropous seed. The 

 embryo is straight and 

 surrounded by endo- 

 sperm. Note the large 

 hypostase at the lower 

 (chalazal) end. {After 

 Sclnnid. ) 



