CHAPTER XXVI 



SEEDS, FRUITS AND SEEDLINGS 



The two primary consequences of fertilization are the development of the 

 embryo and of the endosperm but with these events there are linked a 

 number of other developments which generally do not take place, or do 

 not take place in the same way, unless a viable embryo is present. First 

 in immediacy is the change of the ovule into a seed and secondly, though 

 often contemporaneous, is the change of the ovary into a fruit. Along with 

 these there often go changes in other floral parts: changes in the calyx, 

 changes in the floral axis, even sometimes changes in the involucral bracts; 

 which fit them to take part in the protection or dispersal of the fruits. 



The mature seed is covered externally by a hard or leathery coat, the 

 testa, which is the product of one or both of the integuments. Its colouring 

 and surface texture are extremely variable in different plants but show a 

 recognizable generic and even familial constancy. The position of the 

 micropvle is often evidenced by a slight depression in the testa, but the most 

 conspicuous mark externally is the scar left by the detachment of the 

 funicle, known as the hilum, which is often coloured differently from the 

 rest of the seed. Seeds of Papilionaceae usually have a very long hilum, 

 sometimes extending nearly round the seed, due to the fact that the funicle, 

 instead of merging into the raphe, is flattened and expanded at the top so 

 that it clasps the seed along a wide arc of its circumference. In the case of 

 anatropous seeds, the micropyle is at one end of the hilum and at the ether 

 end, in the seeds of some families, is a small, elongated depression, sur- 

 rounded by a ridge. This is the point traversed by the vascular bundle ot 

 the funicle, entering the raphe. It is important because it constitutes a 

 break in the waterproof coverings of the seed and when it is present is the 

 primary channel of rapid absorption of water before germination. In the 

 majority of seeds the primary penetration of water is through the micro- 

 pyle, but in some hard and resistant seeds there is no special channel of 

 entry, and water can only penetrate if the testa is either softened or mechani- 

 cally damaged. 



The hilum may play a significant part in the loss of water from the 

 ripening seeds. Evaporation is general from the testa until the water con- 

 tent drops to about 15 per cent., when the testa becomes impermeable. At 

 a water content of about 25 per cent, a shrinkage cleft may open along the 

 median line of the hilum and evaporation continues through this cleft until 

 further shrinkage brings its edges together and finally seals the seed against 

 further water loss. 



Hairy coverings on the seed coat are not common, except in Malvaceae, 



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