373 



LEAF. 



LEAF. 



374 



deciduse), as in Liriodendron, or in leaves or stipules of simple struc- 

 ture, of which the laminar portion is abortive (tegmenta) : and there 

 are varieties amongst these, so far that either only the external or 

 inferior leaves, or stipules, appear as coverings of the buds, as in 

 Fagui ; or the coverings of the bud seem to be continued into the 

 interior of the bud, but alternate with leaves capable of perfect 

 development, which lie between and are covered by them, as in Acer. 

 The coverings of the bud are for the most part tough, and almost 

 leathery; they are often filled and coated over with resinous juices, 

 and then mostly fall off in the development of the bud, but they also 

 occur thin and herbaceous in texture, and even change quickly into 

 dry thin membranes, which mostly remain upon the plant ; these last 

 are seen in Pinus. 



6. Adventitious buds of perennial plants, with vegetation periodically 

 dormant. They are only distinguished from the foregoing by the mode 

 of development. Each stem, whethera common one or a root stem, 

 can develop a bud. These buds are caused, not only by accidental 

 and intentional wounding of the stem, but also by the inclination of 

 plants to develop buds at certain places. Many plants exhibit upon 

 their bark peculiar little groups of lax roundish cells, which originally 

 lie under the epidermis, which however is soon destroyed above them, 

 leaving them bare (lenticellce). The result of this exposure is, that 

 at these places the bark is rent by the distension of the bough or stem ; 

 hence the newly vegetating part of the bark comes in contact with 

 the air. It is principally at the edges of the rent bark that the 

 adventitious buds are found. 



2. Propagative Buds. 



n. Bulbs (Bulbi) are monocotyledonous stems, with undeveloped 

 nternodes, which gradually die away from below upwards, and there- 

 fore remain always very short, with perennial leaves, whose vaginal 

 parts die away and inclose thin membranes, the sheaths of the 

 inner leaves still living, and always fleshy and thick (bulb-scales) ; or 

 more rarely die away speedily, and leave bare the latter, as in Lilium. 

 They are formed either immediately from the embryo, and then the 

 sheathing part of the cotyledonary leaf becomes the first bulb-scale ; 

 or they are formed from the axillary buds of the bulb, or from the 

 axillary buds of the stems which have sprung from the bulbs, as in 

 Lilium bidbiferum : less frequently they are from adventitious buds 

 on leaves or other parts. We distinguish : 



A. The Leafy Bulb (Bulbus Foliosus). 



1. The Tunicated Bulb (Bulbus Tunicatus), where many sheathing 

 parts are closed round, or embrace the axis pretty broadly, as in 

 ffyacintkut orientalit. 



2. The Scaly Bulb (Bulbus Squamosus), where many sheathing 

 parts, relatively slender and short, are seated on the axis, as in Lilium 

 candidum. 



B. The Solid Bulb (Bulbus Solidus), when the bulb is formed of 

 one single living sheathing part. 



l>. Bulbels (Bulbilli). To plants not perennial by means of a bulb 

 (only Dicotyledonous ?), the axillary buds are sometimes developed 

 into bulb-like forms, in which the leaves are only developed ag 

 thickened sheathing parts, and the buds separate from the parent 

 plant by the dying away of the supporting stem or stalk, and become 

 independent plants, as with Dentaria bulbifera. 



c. Tubers (Tubera). On underground stems the axillary buds (of 

 attenuated scaly leaves) are sometimes developed in such fashion that 

 the entire axis of the bud becomes thickened, fleshy, and of a knobby 

 form ; the leaves are quite in rudimentary condition, or scarcely to 

 be recognised, whilst the axillary and terminal buds remain capable 

 of development, and after the dying away of the stems of the parent 

 plant form new stems, as in Solanum tubtrotum. 



d. Tuber-Buds, Tubercles (Tubercula). Many plants form small 

 tubers above the earth ; seldom (if ever) indeed as axillary buds, but 

 frequently an adventitious buds, and especially on foliaceous organs, 

 from which new independent plants develop as soon as they are 

 separated from the parent plants : sometimes this is a specific pecu- 

 liarity, an, for instance, the tubers of the species of Amorpliophalha 

 and other Aroidete; sometimes they arise in certain plants particularly 

 readily in consequence of injuries, as, for instance, in the Gesneriacea, 

 on the broken surface, after cutting a leaf-nerve at the edge or the 

 point of the leaf. 



e. Pseudo-Tubers (Tuberidia). In some plants a single, frequently 

 an axillary, bud is transformed in a peculiar manner. The paren- 

 chyma of the axis of the bud, which is situated over the vascular 

 surface, suddenly becomes exceedingly expanded in a solid and 

 tubercn'nt< rl form, by means of the sudden commencement of new 

 formation of cells in isolated groups of cells ; in the axillary bud this 

 only occurs on one side (as in our native Orchidere), since, on the 

 other side, the pressure of the stem prevents such distension. In 

 Aponogeton dittachyon, the thick fleshy cotyledon with the end of the 

 root proves a corresponding obstruction ; hence here also the develop- 

 ment of the pseudo-tuber is only one-sided. In the Dahlias, on 

 the contrary, the tubercular development is equal on both sides. The 

 mas of cells enters between the base of the cotyledon and the new 



adventitious roots, arising at a very early period almost immediately 

 under the cotyledon, and which, through the formation of the pseudo- 

 tubers, become gradually removed faraway from the cotyledon. 



We may now add a few words on the function of the leaves. 

 Works on botany are full of speculations on the functions performed 

 by these organs. Without entering on this history we may say that 

 the whole of the functions performed by the leaves may be summed 

 up in the word exhalation. The sap is brought up from the soil by 

 the loss of water from the leaves. Hence this function of the leaves 

 is necessary to the nutrition of the whole plant. It does not however 

 follow as a consequence of this fact that the leaves send down 

 nourishing materials into the stem and branches. 



It is also frequently stated that the leaves take up carbonic acid. 

 Under certain circumstances there can be no doubt that leaves 

 absorb as well as exhale, but this is not their constant function. It 

 depends on the condition of the atmosphere ; when dry and the sun 

 is shining the leaves exhale water, but if these conditions are not 

 present, if the atmosphere is moist, and there is little or no heat and 

 light, then the leaves absorb. 



The following extract from Schleiden's 'Principles of Scientific 

 Botany" supplies some of the data on which recent physiologists base 

 their view of the leaf being simply an organ of exhalation. 



"From those parts of plants which are exposed to an atmosphere 

 which is not already perfectly saturated with moisture, a continual 

 evaporation of water goes on. The process is purely physical, and, 

 according to accurate investigations, it appears to proceed uninter- 

 ruptedly, according to the dryness and motion of the atmosphere, 

 with the temperature, and the amount of surface exposed to evapora- 

 tion. It is highly probable that the epidermis permits of no passage 

 to the evaporating water, but that, the evaporation occurring in the 

 neighbouring intercellular spaces, it escapes through the stomates 

 when they are not closed by too strong evaporation and consequent 

 relaxation. From this circumstance the exhaled water is never quite 

 pure, but it contains always a small admixture of vegetable substances 

 which cannot be accurately analysed. 



"Besides this evaporation of water, we sometimes find in a very 

 damp atmosphere, and especially in the case of plants that have 

 already exhaled very much, a taking up of moisture, especially through 

 their green parts ; but our observations on this fact have been too 

 little accurate and purposeless to admit of a precise explanation of 

 the process. 



" The study of vegetable exhalation in general requires a repetition 

 and improvement of the experiments made upon it. We need a set 

 of experiments which should show, with the greatest exactitude, the 

 difference between the quantity of water absorbed and exhaled, from 

 which we might decide the quantity used for the nourishment of the 

 plant. If the amount of oxygen exhaled with the water was also 

 obtained, we should probably be able to arrive at conclusions respecting 

 the nature of the chemical processes carried on within the plant. We 

 have yet to ascertain the relation of the exhalation of the water to its 

 absorption. The fact of its absorption (by other means than the root) 

 has been established by Hales, but we are still quite in the dark as to 

 the manner. An accurate knowledge of these relations is so much che 

 more to be desired, as the evaporation and absorption of water with 

 the tension of the vapour must exert an influence upon the absorption or 

 exhalation of the several kinds of gases. Yet in the experiments made 

 upon the so-called respiration of plants, this has been lost sight of. 



" We know nothing of the precise types through which exhalation is 

 effected. To myself it appears improbable that the living epidermis 

 should be permeable to water and the vapour of water, except through 

 the utomates. 



" It is an established fact, that all evaporating water carries with it 

 some portion of the matter which it held in solution. This is seen 

 in the vapour of the ocean. It is probable that no water exhaled 

 from plants is absolutely pure. But no accurate analyses have been 

 made on this point. 



" The natural consequence of this exhalation of water from the 

 green parts of plants which are exposed to the air, is the continual 

 concentration of the juices in the cells which lie next the evaporating 

 surfaces. By this the endosmose of the cells which do not exhale 

 undergoes a change, of which we shall have to speak hereafter. 



" The information which we possess respecting the exhalation of 

 plants is chiefly found in the experiments of Hales, Guettard, 

 Sennebier, Scbubler, and Neuffer. 



" The strange tendency always to attribute to vitality something 

 different from the physical powers, has introduced into the doctrine 

 of the transpiration of plants a distinction between evaporation and 

 exhalation ; the first being supposed to take place iu dead plants and 

 the last in living ones. I can find no distinction in this case in the 

 facts, but merely in the words. 



" I will here add some facts upon the quantity of water exhaled 

 by plants. 



"According to Hales, a sunflower evaporated daily 1 '25 Ibs. of 

 water: now let us allow to each of these plants 4 D ' space of soil; 

 then upon the old Hessian acre there would stand 10,000 plants, 

 which in 120 days would exhale 1,500,000 Ibs. of water. 



" A cabbage exhaled in twelve hours of the day 1 Ib. 6 oz. of water ; 



