PHYSIOLOGY 305 



in the process of fertilisation, are also concerned in removing the 

 inhibitions on the growth of egg-cells. 



Some seeds pass through a prolonged resting period before they commence to 

 germinate. They may lie for years in the soil, while others of the same age have 

 germinated long before ; this in part depends on the hardness of the seed-coat and 

 the consequent difficulty of swelling. This also appears to be the main reason 

 why the seeds of many aquatic plants ( 6e ) will not germinate in pure water, 

 but do so on the addition of acids or alkalies. In some cases fully swollen 

 seeds are unable to germinate except in the LIGHT ( 67 ). The red and yellow rays 

 are usually more effective than more highly refractive rays, and a surprisingly 

 short exposure to illumination may suffice (Lythrum salicaria, -^ second, at 

 Hefner-Kerze intensity of illumination 730). Not uncommonly the illumination 

 may be replaced by a particular high temperature or by chemical stimuli. The 

 latter play the chief part in the case of certain parasites which only germinate 

 in the vicinity of their host plants (Orobanche, Tozzia). In other cases (e.g. 

 AiiKirantus] light hinders or delays germination, and darkness is an advantage. 



In the case of spores also germination may begin on the establishment of the 

 formal conditions of growth or may require special stimuli. 



A striking and fixed resting condition is seen in deciduous trees. 

 At a certain season of the year, in the autumn or earlier, their buds 

 can in no way be induced to expand. Later, however, a considerable 

 shortening of their resting period may be caused not only by a higher 

 temperature but by a number of stimuli such as frost, heat, dryriess, 

 darkness, illumination, ether vapour, acetylene, tobacco smoke, wound- 

 ing, injection of water, etc. . 



The awakening from the resting state ( 68 ) is most readily effected shortly before 

 the normal resumption of activity, but almost as readily at an early period shortly 

 after the period of rest has begun. In the intervening period of complete rest, 

 attempts at removing the inhibition on growth are usually without effect. These 

 relations have to be taken into consideration in the forcing of plants in horti- 

 cultural practice. 



The Stimulus of Restitution ( 69 ). The causes of the commencement 

 of growth in the case of restitutions have also to be considered. The 

 answer appears simple, since the phenomena as a rule follow on 

 wounding. The fact, however, that processes that resemble restitu- 

 tions are met with in the course of normal development shows that 

 circumspection is required. Thus, for example, young plants are 

 developed in the indentations of the leaves of Bryophyllum, and in the 

 case of certain Begonias shoots are developed from the intact as well 

 as from the incised leaf-blade. It has been shown experimentally that 

 for many true restitutions it is not the removal of an organ but 

 the interruption of its functions that is required to start the new 

 growth (p. 297). 



. Polarity. The fertilised ovum of the flowering plant, when the 

 inhibition on its growth is removed, forms two distinct growing 

 points for the shoot and root respectively. A corresponding polar 



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