I40 BOTANY OF THE LIVING PLANT 



.,„,„.„;„„ ()t lll( . I(li | hapter [.), The external conditions 



nducing to germination of the dormant seed are : 



Xhc pr« of moisture, which causes swelling, and the passage 



fr<jm that state of desiccation which its tissues show in the 



dormant seed. The mobilisation of the food reserves soon 



follows (sec Chapter Y1II.). 



(b) Access to air : actually it is the oxygen component that is 



needed for respiration. 



(c) Temperature within the range to which vital activity is re- 



stricted. 



While in a majority of cases light is without effect, in some seeds germination 

 is retarded or even prevented by the incidence of light, while in others light 

 has a benefit ial effect. The retarding effect obtains in the seeds of some 

 Phloxes, the benefii ial one in seeds of Mistletoe {Viscum album), Tobacco and 

 Purpl< .strife [Lythrum salicaria). The full explanation of these effects 



1-, not j < t available 



It does not follow that germination will always occur when the external 

 conditions are favourable. The seed may continue to show dormancy as the 

 result of the operation of some internal factor. Seeds of many species are 

 incapable of germinating immediately after liberation from the parent plant. 

 This may be because the embryo is immature at the time of separation from 

 the parent, as it is in the seeds of the Lesser Celandine (Ranunculus ficaria), 

 Marsh Mangold (Caltha palustris), the Ash (Fraxinus excelsior), and perhaps 

 the Sycamore (A cer psendoplatanus) . When the seeds of such plants are placed 

 under suitable conditions for germination, the development of the embryo 

 is completed, but germination is arrested until that stage is reached. In 

 the Sycamore and Ash germination may be held up in this way for months, 

 and so tide over the winter. In a number of other cases the testa has pro- 

 perties which for a certain time prevent germination. It may show a high 

 degree of impermeability towards water, so that no absorption proceeds 

 even if the seeds are submerged. This is the case in seeds of many members 

 of the Leguminosae, as for example Gorse (Ulex europaeus), Clover (Trifolium 

 spp.) and Sweet Pea (Lathyrus odoratus). The testa gradually becomes 

 permeable to water, but germination may be delayed for months or even 

 years ; there is, however, great variation in this respect among the seeds 

 from a single plant. The advantage of this may be that the germination 

 of the seeds produced by a parent plant is spread over several months or 

 years : the chance of some of the seedlings finding favourable conditions for 

 their development is thereby increased. By chipping the impermeable testas 

 of these seeds prompt swelling and germination can be secured. In other 

 cases the seed coat is at first relatively impermeable to oxygen and carbon 

 dioxide, respiration and hence germination being impeded : in other cases the 

 seed coat restrains the sprouting of the embryo for a time mechanically. 



The period over which seeds retain their capacity for germination, 

 though denied conditions suitable for it, — that is their viability, — 



