Book I. GERMINATION OF THE SEED. 227 



Sect. I. Germination of the Seed. 



1512. Germination is that act or operation of the vegetative principle, by which the 

 embryo is extricated from its envelopes, and converted into a plant. This is univer- 

 sally the first part of the process of vegetation ; for it may be regarded as an indu- 

 bitable fact, that all plants spring originally from seed. The conditions necessary to 

 germination relate either to the internal state of the seed itself, or to the circumstances 

 in which it is placed with regard to surrounding substances. 



1513. The first condition necessary to germination is, that the seed must have reached maturity. Un- 

 ripe seeds seldom germinate, because their parts are not yet prepared to form the chemical combinations 

 on which germination depends. There are some seeds, however, whose germination is said to commence 

 in the very seed-vessel, even before the fruit is ripe, and while it is yet attached to the parent plant 

 Such are those of the Tangekolii of Adanson, and Agave vivipara of East Florida, as well as those of the 

 Cy^amus Nelumbo of Sir J. E. Smith, or sacred bean of India ; to which may be added the seeds of the com- 

 com garden radish, pea, lemon, &c. But these are examples of rare occurrence ; though it is sometimes 

 necessary to sow or plant the seed almost as soon as it is fully ripe, as in the case of the coftee-bean ; 

 which will not germinate unless it is sown within five or six weeks after it has been gathered. Most 

 seeds, however, if guarded from external injury, will retain their germinating faculty for a period of many 

 years. This has been proved by the experiment of sowing seeds which have been long so kept ; as well as 

 by the deep ploughing up of fields which have been long left without cultivation. A field which was thus 

 ploughed up, near Dunkeld, in Scotland, after a period of forty years' rest, yielded a considerable blade of 

 black oats without sowing. This could have only been by the i^lough's bringing up to the surface seeds 

 which had been formerly too deeply lodged for germination. 



1514. The second condition is, that the seeds sown must be defended from the action of the rays of light. 

 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. 



1515. 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. This degree varies considerably in dif- 

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

 climate or in different ones : 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. {Famil/es des Plantes, vol. i. p. 84.) Upon the same principle, 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 exemphfied 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. 



1516. A fourth condition necessary to germination is the 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 

 be too little, the seed dies for want of moisture ; if there be 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 peas, 

 which he placed merely upon a piece of wet sponge, so as to immerse them by neaiiy 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,which will germinate even 

 when wholly submersed. The seeds of aquatics must of necessity germinate under water ; and peas have 

 been known to do so under certain conditions. 



1517. 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 readmission of 

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

 no seed will germinate in nitrogen gas, or carbonic acid gas, or hydrogen gas, except when mixed with a 

 certain proportion of oxygen gas ; and hence concluded that oxygen gas is necessary to the germination 

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

 boldt round 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. 



1518. The period necessary to com])lete 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 : 



Wlieat, Millet-seed 

 Spinach, Beans, Mustard 

 Lettuce, Aniseed 



Melon, Cucumber, Cressl 



seed 



1519. Physical phenomena. When a seed is committed to the soil under the conditions 

 which have been just specified, the first infallible symptom of germination is to be detluced 



Q 2 



