PHY 



OR, BOTANICAL DICTIONARY. 



PHY 



309 





of light. He believes that in this manner the cooling shade 

 of trees may be accounted for. The functions of absorption 

 and exhalation appear to take place in Mushrooms according 

 to their principles. This, however, needs to be confirmed 

 by future observations. Agaricus campestris and androsa- 

 ceus, continually exhale hydrogen. Oxygen appears to be 

 a stimulus to them, as the most of them when immersed in 

 hydrogen and azotic gas are soon destroyed. 



" How the matters which are absorbed by plants are assi- 

 milated, that is, are combined into the juices peculiar to the 

 vegetable world, is a mystery to us. In none of the orga- 

 nized bodies have we hitherto been able to explain this assi- 

 milation, though there has been no want of theories upon 

 the subject. Some account for this beautiful operation by 

 mere contraction of the parts ; others, by the form of the 

 active organs ; others again, by the form of the substances ; 

 but these are all very unsatisfactory hypotheses. This much, 

 in the mean time, appears certain, that the proportion of the 

 parts, as well as the formation and direction of the organs, 

 and the greater or less irritability arising from them, may 

 produce the various mixtures. But how comes it to pass, 

 that every part of a plant frequently differs in taste and in 

 smell? Thus the root of the Mimosa Nilotica smells like 

 Assafotida, but the flower emits a very agreeable odour. 

 The stem exudes the bland well-known gum arabic, and the 

 juices which it contains are sour and astringent. Manure 

 operates only as a stimulus on the fibre of vegetables, so 

 that they are enabled to absorb carbon the more rapidly, 

 and all the constituents are first composed. This is parti- 

 cularly evident from the above related experiments, where 

 delved earth, saturated with oxygen, as well as ground 

 sprinkled with, diluted sulphuric acid, made plants grow 

 more rapidly than a great deal of manure laid upon the 

 earth. The vessels appear particularly to prepare the juices, 

 as fluids have been found completely prepared in the cellular 

 texture of the root. It seems, however, as if the receptacle 

 of the manures in the cellular texture concentrated them still 

 more, and that this also contributes its effect to their pre- 

 paration. Thus as glands prepare in general oily, seldom 

 mucilaginous fluids, a great quantity of glands is therefore 

 found in the leaves of all fragrant plants. Sometimes this 

 oil abounds so plentifully in the glands, that it may be sepa- 

 rated from them by mere pressure ; as in the rind of Citrus 

 medica, aurantium, &c. and in the leaves of Melaleuca leu- 

 codcndron. Vessels which are still young, are the most 

 active in plants. As soon as they begin to pass into albur- 

 num or wood, the circulation of their fluids is in a sensible 

 degree slower. Thus the chief seat of life, particularly in 

 ligneous plants, is to be sought for in the inner bark. Hence 

 trees become strong and large when they receive no external 

 wound on the stem, so as to injure the inner bark. Trees, 

 the bark of which was frozen in. severe winters, will decay, 

 whereas those which lose their pith by cold, without their 

 inner bark being affected, may continue to grow without 

 sustaining any injury. Where the layer of the inner bark, 

 which as we know is composed of vessels, is thinnest, the 

 growth is most rapid, and leaves are also formed. For this 

 reason, thin branches are provided only with leaves. The 

 inner bark begins in the alburnum, i. e. there are new vessels 

 situated in the alburnum, which as long as they form a thin 

 delicate layer easily separable, are called inner bark. This 

 layer, however, hardens into alburnum, and at last into 

 wood. Duhamel saw no connection between the cortex and 

 wood of a Willow-tree. He found, however, a moisture, 

 which in the air became mucilaginous and tenacious; this 

 he, with Grew, called Cambium. He alleges that it is the 



formative organ in plants. He took away from a Cherry- 

 tree in full bloom all its cortex longitudinally, and covered 

 it closely with a layer of straw; many of the leaves fell off, 

 some of the branches withered, and no fruit was produced. 

 The tree continued diseased next year, but in the third sum- 

 mer it again acquired bark. Had the young wood, deprived 

 of the stem which is full of moisture, and which had lately, 

 for the first time, formed the new layer of inner bark, not 

 been preserved from the access of the air, the juice would 

 have dried up, and the tree been destroyed ; but the covering 

 of straw inclosed it as well as the cortex, and it again formed 

 the same layer of inner bark with the cortex. The hardened 

 fibres of vegetables, comprehended under the general name 

 of Wood, have, however, different degrees of hardness in 

 proportion as they confine carbon by the power of their 

 organization, and the harder the wood proves, the slower is 

 the growth of the tree or shrub. The firmest and hardest 

 woods, have, therefore, the most carbon, and require a long 

 time for their perfect vegetation; as, the White Beech, 

 (Carpinus betulus,) the Red Beech, (Fagus sylvatica,) the 

 Oajc, (Quercus robur and pedunculata,) the Cedar of Leba- 

 non, (Pinus cedrus,) Adansonia digitata, &c. &c. There 

 are, however, exceptions to this rule; as Robinia pseudacacia, 

 which grows very fast, and has firm hard wood. Every 

 shrub or tree with us, forms annually two shoots; the one, 

 which is the chief shoot, evolves in spring, the other is not 

 so strong, and appears towards the longest day, about St. 

 John's day, from which it has been called St. John's shoot. 

 The first is formed from the quantity of juices which the root 

 has imbibed during winter. The second, from the moisture 

 imbibed during the spring. In the torrid zone, both shoots 

 are equally strong, and hence plants there grow much more 

 luxuriantly. 



" The green colour "of the vegetable creation is a most 

 refreshing sight. The investigation of its cause has long 

 occupied the attention of philosophers, and given rise to 

 many hypotheses. When phlogiston still had a number of 

 adherents, the explanation of the green colour was very 

 easy, as it was considered as an effect of this principle. 

 Since, however, the idea of its existence has been given up, 

 different kinds of explanation have been devised. Berthollet 

 observed, that the green of plants is not composed of blue 

 and yellow, as the prism does not analyze their green, like 

 that of other bodies, into yellow and blue rays. After 

 extracting with alcohol the green colour from the leaves, 

 and exposing this mixture to the sun or atmosphere, the 

 green colour disappears entirely. The oxygen of the atmo- 

 sphere combines with the mixture, and banishes the colour. 

 If a solution of ammonia, which consists of hydrogen and 

 azote, be dropped into it, the latter separates the oxygen 

 from the mixture, and the green colour is restored to it. 

 From all the observations on this point, it follows, that 

 leaves, from which the oxygen has been withdrawn by means 

 of light, are green, but have a pale or whitish colour where 

 the oxygen is accumulated. The mixture of hydrogen and 

 carbon is now considered by chemists as the cause of the 

 green vegetable colour. The dark colour of the cortex in 

 woody plants is, according to Berthollet's observations, pro- 

 ducctl by the oxygen of the atmosphere. Mr. Humboldt 

 repeated his experiments, and found that wood, when enclosed 

 in oxygen gas, became black in two or three days, and the 

 gas was mixed with carbon. It appears from this, that the 

 oxygen of. the atmosphere .combines with the hydrogen of the 

 vegetable fibre, and sets the carbon free, which s-hews its 

 particular black colour. 



" The leaves of plants are of various duration; Most of 



