Book I. 



GERMINATION OF THE SEED. 



227- 



black oats without sowing. It could have been only by the plough's bringing up to the surface, seeds that 

 had been formerly too deeply lodged for germination. 



1488. The second condition is, that the seeds sown must be defended from the action of the rays of tight. 

 This has no doubt been long known to be a necessary condition of germination, if we regard the practice 

 of the harrowing or raking in of the grains or seeds sown by the farmer or gardener as being founded 

 upon it. 



1489. A third condition necessary to germination is the access of heat. No seed has ever been known 

 to germinate at or below the freezing point. Hence seeds do not germinate in winter, even though 

 lodged in their proper soil. But the vital principle is not necessarily destroyed in consequence of this 

 exposure ; for the seed will germinate still, on the return of spring, when the ground has been again 

 thawed, and the temperature raised to the proper degree. But this degree varies considerably in dif- 

 ferent species of seeds, as is obvious from observing the times of their germination, whether in the same 

 or in diiferent climates. For if seeds which naturally sow themselves, germinate in difFerent climates at 

 the same period, or in the same climate at different periods, the temperature necessary to their germi- 

 nation must of consequence be different. Now these cases are constantly occurring and presenting them- 

 selves to our notice ; and have also been made the subject of particular observation. Adanson found 

 that seeds which will germinate in the space of twelve hours in an ordinary degree of heat may be made 

 to germinate in the space of three hours by exposing them to a greater degree of heat ; and that seeds 

 transported from the climate of Paris to that of Senegal, have their periods of germination accelerated 

 from one to three days. {Families des Plantes, vol. i. p. 84.) Upon the same i)rinciple, seeds transported 

 from a warmer to a colder climate, have their periods of germination protracted till the temperature of 

 the latter is raised to that of the former. This is well exemplified in the case of green-house and hot- 

 house plants, from which it is also obvious that the temperature must not be raised beyond a certain 

 degree, otherwise the vital principle is totally destroyed. 



1490. A fourth condition necessary to germination is th^ access of moisture. Seeds will not germinate 

 if they are kept perfectly dry. Water, therefore, or some liquid equivalent to it, is essential to germi- 

 nation. Hence rain is always acceptable to the farmer or gardener, immediately after he has sown his 

 seeds ; and if no rain falls, recourse must be had, if possible, to artificial watering. But the quantity 

 of water applied is not a matter of indifference. There may be too little, or there may be too much. If 

 there is too little, the seed dies for want of moisture ; if there is too much, it then rots. The case is not the 

 same, however, with all seeds. Some can bear but little moisture, though others will germinate even 

 when partially immersed ; as was proved by an experiment of Du Hamel's, at least in the case of pease, 

 which he placed merely upon a piece of wet sponge, so as to immerse them by nearly the one-half, and 

 which germinated as if placed in the soil. But this was found to be the most they could bear ; for when 

 totally immersed in the water they rotted. There are some seeds, however, that will germinate even 

 when wholly submersed. The seeds of aquatics must of necessity germinate under water j and pease 

 have been also known to do so under certain conditions. 



1491. A fifth condition necessary to germination is the access of atmospheric air. Seeds will not germi- 

 nate if placed in a vacuum. Ray introduced some grains of lettuce-seed into the receiver of an air-pump, 

 which he then exhausted. The seeds did not germinate. But they germinated upon the re-admission of 

 tiie air, which is thus proved by consequence to be necessary to their germination. Achard proved that 

 no seed will germinate in nitrogene gas, or carbonic acid gas, or hydrogene gas, except when mixed with a 

 certain proportion of oxygene gas ; and hence concluded that oxygene gas is necessary to the germinatiori 

 of all seeds, and the only constituent part of the atmospheric air which is absolutely necessary. Hum- 

 boldt found that the process of germination is accelerated by means of previously steeping the seed in 

 water impregnated with oxymuriatic acid. Cress-seed treated in this manner germinated in the space 

 of three hours, though its ordinary period of germination is not less than thirty-two hours. 



1492. The period necessary to complete the process of germination is not the same in all 

 seeds, even when all the necessary conditions have been furnished. Some species require 

 a shorter, and others a longer period. The grasses are among the number of those plants 

 whose seeds are of the most rapid germination ; then perhaps cruciform plants ; then 

 leguminous plants ; then labiate plants ; then umbelliferous plants ; and in the last 

 order rosaceous plants, whose seeds germinate the slowest. The following table in- 

 dicates the periods of the germination of a considerable variety of seeds, as observed by 

 Adanson : 



Davs. Days. Days. 



Hyssop .... 30 ^ 



Parley - - - - 40 or 50 

 Almond, Chestnut, Peach 1 year 

 Rose, Hawthorn, Filbert . 2 years. 



1493. Physical phenomena. When a seed is committed to the soil under the conditions 

 that have been just specified, the first infallible symptom of germination is to be deduced 

 from the prolongation of the radicle (Jig. 230 a.; ^ 230 

 bursting through its proper integuments, and direct- 

 ing its extremity downwards into the soil. The next 

 step in the process of germination is the evolution of 

 the cotyledon or cotyledons (c), unless the seed is al- 

 together acotyledonous, or the cotyledons hypogean, 

 as in the oak i b). The next step, in the case of seeds 

 furnished with cotyledons, is that of the extrication 

 of the plumelet (c), or first real leaf, from within or 

 from between the cotyledon or cotyledons, and its 

 expansion in the open air. The last and concluding 

 step is the developement of the rudiments of a stem 

 (rf), if the species is furnished with a stem, and the 

 plant is complete. Wliatever way the seed may be 

 deposited, the invincible tendency of the radicle is to 

 descend and fix itself in the earth ; and of the plumelet to ascend into the air. Many 

 conjectures have been offered to account for this. Knight accounts for it on the old 

 but revived principle of gravitation. Keith conjectures that it tajces place from a power 



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