'ill 



EXOGENS. 



EXOGENS. 



(U2 



power of the first spring or leap they make on leaving their nativ 

 element." 



Judging from the foregoing quotation, and several other account 

 which we have perused, it would appear that something beyond th 

 mere leap of the fish would be required to account for the grea 

 heights (of 14 or 20 feet) at which these fishes have been seen. I 

 they cannot fly (which one would judge to be the case upon examinin 

 the structure and position of the fins), it seems probable that the 

 take advantage of the wind at times, and so adjust their fins that the 

 are carried upward by it. 



Several instances are on record of the appearance of Flying Fishe 

 off the British coast, but the species is doubtful. It is probable tha 

 both the Exocetus exiliem and the E. volitam may have made thei 

 appearance in our seas ; these two species being very abundant, th 

 former in the Mediterranean Sea (where many fishes similar to thos 

 of our own coast occur), and the latter in the Atlantic Ocean. 



The E. exiliens has the ventral fins placed behind the middle of the 

 body, and the E. volitana has the ventrals (which are much smalle 

 than in E. e;rilient) placed anterior to the middle part of the body 

 these two species therefore are easily distinguished ; of the latter there 

 is a figure in Tan-ell's ' British Fishes.' 



The American seas afford us examples of other species of thi. 

 genus. 



)GENS, the largest primary class in the vegetable kingdom 

 are so named in consequence of their woody matter being augmentet 

 by additions to the outside of that which is first formed near the centre 

 As long as they continue to grow they add new wood to the outside 

 of that formed in the previous year, in which respect they differ 

 essentially from Endogens, whose wood is constructed by successive 

 augmentations from the inside. [EXDOOENS.] All the trees of cole 

 climates, and the principal part of those in hot latitudes, are 

 exogenous. In many cases they are easily recognised by the wooc 

 of each different year forming a distinct zone, so that a section ol 

 their wood exhibits a number of concentric circles ; but there are so 

 many exceptions to this rule as to render it necessary to consider 

 this character as by no means essential to them. 



The nature of the exogenous mode of growth will be best compared 

 with that of an Endogen, if we pursue the same mode of illustration 

 as in the article which treats of the latter form. We will therefore 

 proceed from an explanation of the typical mode of growth in a 

 common Exogen to such remarks as we may have to offer upon 

 deviations from it. 



In mi Exogen of ordinary structure the embryo consists of a 

 cellular basis, in which there is usually no trace of woody or vascular 

 tissue; but as soon as germination commences fine ligneous cords 

 are seen proceeding from the cotyledons towards the radicles from 

 the opposite sides of the young stem, meeting in the centre of the 

 embryo, and forming a thread-like axis for the root. As the parts 

 grow the ligneous cords are increased in thickness and number, and 

 having been introduced among the cellular basis of the embryo, are 

 separated from each other by a portion of the cellular substance, 

 which continues to augment both in length and breadth as the woody 

 cords lengthen. By degrees the plumule or rudimentary stem becomes 

 organised, and having lengthened a little, forms upon its surface one, 

 two, or more true leaves, which gradually expand into thin plates 

 of cellular substance traversed by ligneous cords or veins converging 

 at the point of origin of the leaves. If at that time the interior of 

 the young plant is again examined, it will be found that more 

 ligneous cords have been added from the base of the new leaves 

 down to the cotyledons, where they have formed a junction with the 

 first wood, and have served to thicken the woody matter developed 

 upon the first growth. Those ligneous cords which proceed from 

 the base of the leaves do not unite in the centre of the new stem, 

 there forming a solid axis, but pass down parallel with the outside, 

 and leave a small space of cellular tissue in the middle ; they them- 

 selves being collected into a hollow cylinder, and not uniting in the 

 middle until they reach that point where the woody cords of the 

 cotyledons meet to form the solid centre of the root. Subsequently 

 the stem goes on lengthening and forming new leaves : from each leaf 

 there may be again traced a formation of woody matter disposed 

 cylindrically as before, and uniting with that previously formed, a 

 cylinder of cellular substance being left in the middle ; and the solid 

 woody centre of the root proceeds in its growth in a corresponding 

 ratio, lengthening as the. stem lengthens, and increasing in diameter 

 as the leaves unfold and new woody matter is produced : the result 

 of which IK, that when the young Exogen has arrived at the end of 

 ita first year's growth it has a root with a solid woody axis, and a 

 tem with a hollow woody axis surrounding cellular tissue, the whole 

 being covered in by a cellular integument. But as the woody cords 

 are merely plunged into a cellular basis, the latter passes between 

 Hi' in in a radiating manner, connecting the centre with the circum- 

 ference by straight passages, often imperceptible to the naked eye, 

 lint, always present. The following diagram illustrates this. 



H'-i" in hurt the origin of pith in the central celluar tissue of the 

 stem, of wood in the woody axis, of bark in the cellular integument, 

 and of medullary processes in the radiating passages of cellular 

 tissue connecting the centre with the circumference. 



The woody axis !R not however quite homogeneous at this time. 

 KAT. HrST. DIV. VOI,. II. 



That part which is next the centre contains great numbers of vessels 

 of different kinds, particularly dotted vessels (vasiform tissue) ; the 

 part next the circumference is altogether destitute of vessels, and 

 consists of woody tissue exclusively : of these two parts that with 

 the vessels belongs to the wood, properly so called, and serves as a 

 mould on which future wood is added ; the other belongs to the 

 bark, separates under the form of liber, and in like manner serves as 

 a mould upon which future liber is disposed. 



Boot. 



Stem. 



At the commencement of a second year's growth the liber separates 

 spontaneously from the true wood, a viscid substance called Cambium 

 is secreted between them, and the stem again lengthens, forming 

 new leaves over its surface. The ligneous cords in the leaves are 

 prolonged into the stem, passing down among the cambium, and 

 adhering in part to the wood and in part to the liber of the previous 

 year, the former again having vessels intermingled with them, the 

 latter having none. The cellular tissue that connected the wood and 

 liber is softened by the cambium, and grows between them horizontally 

 while they grow perpendicularly, extending to make room for them, 

 and consequently interposed between the woody cords of which 

 they each consist, forming in fact a new set of medullary processes 

 terminating on the one hand in those of the first year's wood, and on 

 the other in those of the first year's liber. This addition of new 

 matter takes place equally in the stem and in the root, the latter 

 extending and dividing at its points, and receiving the ends of the 

 woody cords as they diverge from the main body. The following 

 diagram illustrates this, and shows, when compared with the last, 

 what difference there is in the appearance of the stem of an Exogen 

 one and two years old. 



Hoot. 



Stem. 



And thus, year after year, the Exogen goes on, forming zone upon 

 zone of wood, which is permanent, and zone within zone of liber, 

 which perishes as the stem increases in diameter. [BARK.] 



If this account is compared with that given of Endogens, it must 

 ie obvious that the stem of these two great classes is formed from 

 he very beginning in an essentially different manner. Endogens have 

 10 cylindrical column of pith ; their woody arcs are never collected 

 nto a cylinder, through the sides of which the cellular tissue passes 

 n the form of medullary processes ; and the woody matter of their 

 >ark, so to call their cortical integument, is not parallel with that of 

 he wood and spontaneously separable from it : not to speak of 

 mportant anatomical differences, or of the concentric arrangement 

 ventually assumed by the wood of Exogens. In both Exogens and 

 'ndogens a cellular substance is the basis of the whole structure, and 

 xtends horizontally wherever it is necessary to do so ; and in certain 

 Exogens woody arcs, stated to be like those of Endogens, ara found 

 i the pith. These cases properly belong to anomalous forms, but 

 evertheless may be noticed here, in consequence of their direct con- 

 ection with this branch of the subject. One case is that of Zamia ; 

 ut as that genus now belongs to the new class of Gymnosperms and 

 ot to Exogens proper, it need not be considered here. The other 

 ases are Piper, Nyctaginaceous plants, and some others. Professor 

 chultz states (' Natiirliches System des Pflanzenreichs,' p. 320, &c.) 

 lat in Piper, Mirabilis, and Soerhaavia, the central part of the stem 

 onsists of cellular tissue, amongst which cords of spiral vessels and 

 roody tissues are placed either without order, or (in Jtoerhamia) in a 

 ruciate manner, as in Tree-Ferns, and that on the outside of this the 

 oody bundles are arranged circularly into a cylinder. A similar 

 ;atement had long previously been made by Mirbel, who ascribes to 

 Hrabilit and some Umbelliferous plants longitudinal vessels in the 

 ith (' El<!m. do Physiol. Veget./ i. 112), and by Professor Meyer, who 

 nds the pith of Mirubilis lonyiflora, M. dichotoma, Eoerhaaria 

 candew, and O.fybaphus C'ervantesii abounding in many large bundles 

 "sniral vessels within the woody radiated zone. (' De Huuttuvniu 



2 T 



