BOTANY. 



47 



cotyledons. The cotyledons are composed of cellular 

 network, resembling small bladders curiously orga- 

 nised. This network contains the albuminous matter 

 already mentioned, though more concrete, and is 

 supposed to nourish tin- young plant in the earlier 

 stages of its existence. Mirbel considers them as 

 imperfect leaves, and assigns the following reasons 

 for his opinion : When the albumen * is exterior to the 

 substance of the cotyledon!, they resemble leaves very 

 strongly in point of structure ; they are covered on 

 their inferior surface with brown spots resembling 

 those on leaves; they follow the same rule with the 

 leaves on their relative positions. Thus when the leaves 

 are articulated, they are also articulated. The leaves 

 of most monocotyledons truike complete sheaths round 

 the stem, and in these sheaths all the parts are contain- 

 ed ; and when the leaves do not envelope, membranous 

 sheathes are found at the base of the stem, which are 

 only imperfect leaves. These reasons do not appear 

 altogether conclusive, although M. Mirbel thinks 

 them so, particularly if Mrs Ibbetson's observations 

 on the chesnut be correct, from which she has been 

 led to infer, that the chief and sole use of the cotyle- 

 dons is to screen the first leaves from the light and 

 air until they are enabled to bear so powerful a stimu- 

 lus. Willdenow describes three varieties of cotyle- 

 dons in germinating seeds : 1. When the cotyledons 

 grow out of the earth and assume the appearance 

 of leaves, these are commonly called dicotyledons : 

 a very common example is the Pliaseolus Vulgaris. 

 2. When the plumula alone appears above the earth, 

 as in the Pisum sativum. 3. When the two halves 

 of the seed are not separated, but are pushed above 

 ground, and on their side the plumula is evolved, as 

 in the Junci, &c. 



He has divided all corcles into five varieties ; the 

 difference of which chiefly consists in the mode of 

 their bursting. 



1. Dermoolastce, such as have membranous cotyle- 

 dons, which burst irregularly. This is in Fungi. 



2. NemaLlastce. Those in which the cotyledons 

 divide into two halves, and burst into filaments, as in 

 the Algcv. 



3. Pteieoblastcc. When the cotyledons appear 

 above ground divided, and are changed into leaves 

 differing from all the other leaves. 



4. GeoUastce. When the cotyledons remain be- 

 low ground. 



5. Spharoblasta: Are those plants the cotyledons 

 of which come out of the ground in the form of 

 small globules fixed on a stalk, and have the plu- 

 rr.nla besides them, as in Juucim biifonius, subverticitta- 

 tu*. 



Nature has not left the seeds of plants destitute of 

 the means of insuring the existence of their species. 

 Many of them are contained within impenetrable cap- 

 sules; others are guarded by sharp spinous processes; 

 (Fig. 89. Plate Ixx.) and others, again, emit a re- 

 markably foetid odour. But even though thus se- 



cured against external violence, the propagation of Vegetable 

 tlir plant would be limited to the spot on which the - 



parent stock first grew, were the seeds not conveyed 

 to various situations, by, several me, ins with which 

 they are provided. Many seeds are furnikhed with 

 an elastic pericarp, (Fig. 95. Plate Ixx.) which, on 

 bursting, projects its contents to a considerable dis- 

 tance. In this number may be reckoned the wild 

 cucumber (cucumi.isativus), and many more. Near- 

 ly all the seeds of compound flowers are furnish- 

 ed with pappi, by means of which they are waft- 

 ed to a considerable distance by the wind. Different 

 kinds of birds, also, promote the propagation of 

 plants, by swallowing the. seeds, and afterwards void- 

 ing them in an undigested state. The same thing 

 happens to some of the larger animals. Gerardin 

 also states, that the magazines of seed laid up by 

 some insects, also contribute to this end. The waves, 

 no less than the winds, convey the seed from place to 

 place. Besides these, numerous other less general 

 means are employed by nature to maintain the dif- 

 ferent species, a detail of which cannot be made with 

 much advantage. 



When a seed is placed in a situation in which it Germina- 

 has a due proportion of moisture, heat, and air, it tion. 

 absorbs water, produces certain chemical changes on 

 the air, shoots forth a rostel and plumula, and is then 

 said to germinate. The processes of germination 

 cannot proceed for any time, if the seed be excluded 

 from water, heat, and air ; but, on the contrary, 

 when present, the rostel of Linnzus, and the radicle 

 of Grew, shoots forth, and forms the whole root, 

 (Part I, Art. iv.) Fig. 1. Plate Ixxi. The plumula 

 of Linnaeus is next evolved ; the exterior integuments 

 of the seed burst and decay ; the cotyledons, accord- 

 ing to Dr Thomson, become sugar; and the plant 

 gradually assumes a more perfect form. 



Various experiments have been made on the soils SoiK 

 in which plants will grow ; and though it seems that 

 every plant affects some particular soil, yet all seeds 

 will germinate and flourish, to a certain extent, in 

 any material in which all the conditions above stated 

 can be obtained. Thus, it is well known, that sallad 

 plants will grow on flannel. Sukkow made them al- 

 so grow in pounded fluat of lime and barytes. M. 

 Humboldt succeeded in inducing vegetation in me- 

 tallic oxides, in sulphur, in powdered coal, and in 

 various other matters. 



A certain portion of heat is also requisite ; and al- Heat, 

 most every plant requires a different quantity. It is, 

 however, worthy of remark, that although many seeds 

 will grow after having been frozen, yet none have been 

 ever made to germinate at or below the freezing point. 



The absorption of water is an indispensible part of 

 the germinating process. It is effected partly through 

 thepores of the exterior membranes, and partly through 

 the small aperture in the hilrnn. In the first instance, 

 it appears to be a mere mechanical operation, which 

 Mr Ellis, in his work already alluded to, properly 



* The albumen constitutes the bulk of some seeds, as in grasses, corn, palms, lilies, the sole office of which is to nourish the 

 embryo plant until the root be formed. In such cases it is wholly independent of the cotyledons It is said to be wanting in 

 some plants, as in the cucumber tribe, though it is highly probable that it has only not been hitherto detected. 'The albumen 

 of the nutmeg is remarkable both for its vngulurly variegated appearance, and its odour. The i-itilliis is always situated be- 

 tween the albumen and the embryo, wli.-n the former is present, and is supposed to perform analogous functions. For the 

 fullest examination of this suflktance, we are indebted to the justly celebrated Gacrtncr. 



