1504 A TEXTBOOK OF THEORETICAL BOTANY 



There is a fundamental difference between the culture of mature embryos 

 with storage cotyledons and that of minute, immature embryos. The 

 former can often be developed successfully, if they have been allowed a full 

 period of after-ripening, on a purely inorganic growth medium solidified 

 with agar. The latter are heterotrophic and must receive organic nourish- 

 ment in addition to the mineral requirements. Sucrose has generally been 

 found to be the best carbon source, closely followed by maltose. As 

 sources of nitrogen, amino-acids and amides are essentially important. 

 Artificial mixtures of amino-acids and the natural mixture in casein hydro- 

 lysate have been used with good effect, but some, at least, of the acids 

 included in these mixtures are unsuitable and have an adverse effect 

 although their action may be antagonized by other acids present. With 

 Orchid embryos, arginine alone supported good growth, but in Datura 

 no incomplete mixture or single amino-acid equalled the effect of casein 

 hydrolysate or the complete mixture of its constituent acids. Experiments 

 with CapseUa showed that the amide glutamine gave better growth than a 

 complete amino-acid mixture of equivalent nitrogen content. Asparagine 

 was ineffective with CapseUa though it is abundant in seedlings of Lupinus 

 and Pisum, as glutamine is in Brassica. Evidently species differ in regard 

 to their amide requirements. 



Early workers on embryo cuhure, having discovered deficiencies in 

 their media, took refuge in the addition of " natural " fluids, especially 

 coconut milk, w^hich proved very successful with some species but not with 

 others. The effect was attributed to an " embryo factor " in the milk, but 

 later work has shown that it is not essential and probably owes its efficiency 

 to its amino-acid content rather than to any hormone. Auxin does not 

 seem to be essential but has a small positive effect in low concentrations. 

 The type of growth in cultured embryos often follows a different pattern 

 from that in normal development and there is some evidence that this can 

 be modified by variations in the osmotic potential of the medium. 



The culture of pro-embryos has not yet been accomplished. Possibly 

 physiological gradients or as yet unknown substances are influential in the 

 earliest stages. 



The dormancy of seeds is a physiological condition to which we shall 

 return in the next volume of this work, but it has very important biological 

 aspects. Dormancy may be due to a variety of causes, such as extra hard 

 coats; or the need for a period of internal chemical change, known as after- 

 ripening; or the lack of oxygen; or the presence of inhibitory substances, 

 or to other reasons. It is often associated with a low water content in the 

 seed but some seeds will remain dormant even when lying in a moist 

 medium, like the soil, and fully imbibed with water. However it arises, 

 dormancy can often be of value to the plant in passing through unfavourable 

 seasons and in allowing time for w^ide dispersal of the seed. 



The seeds of the principal crop plants have usually only a short period 

 of obligate dormancy, for strains with prolonged dormancy are a nuisance 

 to the cultivator and have been eliminated by human selection through the 



