in their course to the retina, in ordinary vision, do in reality occur, it is in so limited a degree as to occasion no inconvenience. Those physiologists, therefore, who consult the sense alone, are of opinion that the achromacy of the eye is perfect; and it has been attempted to explain this achromacy on the principle of the construction of the achromatic telescope, which indeed it is supposed to have suggested. But the idea that the humours of the eye are so co-adapted, that the dispersion produced by one is corrected by a contrary dispersion occasioned by the other, is probably erroneous. In the achromatic glasses, the dispersion induced, by a convex lens, is remedied by a similar but contrary dispersion effected by a concave lens, -or, at least, the principle thus stated is secured. In the construction of the eye, however, the rays, in their course to the retina, appear only to undergo successive convergencies, at least by refraction; and consequently the degree of dispersion is also augmented successively, at each transition of the rays of light from one humour to another.

There is a part of the eye, however, the action of which has not perhaps been fully ascertained. The iris is supposed to regulate the quantity of light admitted to the retina, and in vision at near distances, to exclude those rays which would otherwise fall with too great obliquity on the crystalline lens. But are there not other effects of the iris, not sufficiently adverted to, in the inflection and dispersion of the rays of light at the edges of this part of the eye ?-and may not these effects be similar to the operation of the concave lens, in the achromatic eye glass? A small perforation in a card induces an inflexion and a dispersion of the rays of light which pass through it; may not the finely fringed edge of the iris induce these changes in a still greater degree? Those coloured rays of light which are most refrangible, are also the most inflectible. Now when the light is intense, or when any divergent rays of light strike the eye, and the eye is so conformed as to induce great convergency of the rays by refraction, and consequently, when the dispersion of the light must be great and very obvious, the pupil is then most contracted

and the inflective and dispersive effect of the iris, greatest. May not this effect of the iris counteract the dispersion of the rays of light induced at their refraction by the humours of the eye? And may not this operation of the iris thus ensure, in ordinary vision, the achromacy of the eye?

On this supposition we should conclude that whenever the disposition of the humours of the eye, and whenever the size of the pupil, was not in just proportion mutually and relatively to the intensity and direction of the rays of light, an unconnected dispersion of the rays of light would occur, and the eye would cease to be achromatic. Is not this in effect the case, in the experiments which have been detailed in this Paper, in which a manifest uncounteracted decomposition of the rays of light actually occurred?

ART. VII. On Cryptogamous and Agamous Vegetation. From the French of C. F. BRISSEAU MIRBEL.

THE subject of cryptogamous and agamous vegetation has

been purposely reserved for a separate Section. The relation between the floral organs of this part of the vegetable creation, and those of the phænogamous part, has not yet been demonstrated in a way to justify the combination of the facts which belong to the one, into the same point of view with those which belong to the other.

In cryptogamous plants, the floral organs are extremely minute, of very distinct forms from those of phænogamous ones, and are often concealed from our sight by peculiar integu

ments.

In agamous plants, either there are no floral organs at all, or else they are of a nature that has eluded the research of the naturalist up to the present hour.

In the plants of this lower degree in the scale of organization in the vegetable creation, propagation is carried on by suckers, bulbs, propagula, and seminula. The two first modes being equally appropriate to the phænogamous plants, and familiar to

every one, do not require any particular notice in this place. The two last are those to which we shall here turn our attention.

Propagula are peculiar to the agamous division. They show themselves in the from of a powder on the surface of the plant; are at no period enclosed within a germen; and have been deemed, with great appearance of probability, mere fragments of the external textures of the vegetable. The races of entire genera are continued by these means alone.

Seminula are common to both the agamous and cryptogamous divisions. They are minute organic bodies, which reproduce the species, and possibly differ from the seeds of the phenogamous division only in the smallness of their volume. Cryptogamous seminula are evolved from germens that form a constituent part of a real pistil. Agamous seminula are developed in conceptacles, a sort of germens, which having never formed any part of genuine pistils, offer no trace of either style or stigma. These diminutive seeds lie sometimes loose in the cavity of their conceptacles, or are at others confined several together in elytræ, a kind of partial conceptacles contained within a common one, which may be considered in this case as the involucre.

The term agamous is of very recent date. From the time of Camerarius, who first demonstrated the sexes of plants, down to a very late period, botanists were divided, into those who admitted the existence of sexes in no plant whatever, and into those who maintained that no species in the vegetable department of the creation was without them. Exclusive views, like these, have their rise in the tendency of the human mind, to draw general conclusions from partial facts, and which is ever the strongest in regard to those points, concerning which we know the least.

The philosophy of Linnæus was far from being untainted with prejudice, any more than that of so many others. Instead of sifting and discussing the theories of those who preceded him, he laid it down as an axiom, that the law of regeneration in vegetables was necessarily the same throughout the whole system. It was he that devised and brought into use the term cryptoga

mous plants. He applied it indiscriminately to those species where he obtained some indistinct view of sexual organs, and to others where he never had the slightest glimpse of any. His doctrine was, that every organized being is endowed with the faculty of propagating itself either by egg or by seed; that an egg or a seed could not be produced without impregnation; and consequently no organized being is destitute of male and female organs, though these may not be discernible by the eye of the observer. Subsequent investigations have however led to the opinion, that there do exist organized beings that produce neither egg or seed; and that others possess these means of multiplying themselves independently of previous impregnation; and most botanists of the present day agree, that the presence of sexual organs in many species included by Linnæus in his cryptogamous class of vegetables, is any thing but proved.

If we adopt the latter opinion, which appears to rest upon solid ground, it follows, that we should divide the vegetable system into three primary divisions, instead of two: the first comprising the phænogamous species, where the process of fecundation is palpable; the second, the cryptogamous ones, where that process is involved in some degree of obscurity; the third, the ones, in which no such process takes place.

agamous

Equal to the facility we may experience in forming a conception of three such classes in a general point of view, is the difficulty we shall find in determining the boundaries of each with precision. Experiment is undoubtedly the most direct test of the presence of the sexes. We can be at no loss concerning the office of the stamens, when we perceive that the ovula constantly miscarry in the most perfectly constituted germen, if the pollen has not reached the stigma; and on the other hand, find that the same as unfailingly come to perfection wherever the pollen arrives at the destined point.

A less direct test, but one that may be as safely depended upon, is analogy, and it is better suited to our daily purposes. We admit without hesitation, a multitude of plants into the ranks of the phænogamous species, upon which it has never entered our head to try any direct experiments in regard to the powers of

fecundation, being satisfied, by a comparison of the orginization of their flowers with that of the few in which the existence of the sexes have been directly demonstrated, that we are entitled to acknowledge their being qualified with stamens and pistils. But in cases, where on one hand the analogy of the organs, grounded upon the similitude of forms, is not clear; on the other, the structure and the minuteness of the parts preclude the possibility of the test by experiment; it is easy to conceive, that the existence of the sexes may become a question; and this is what has actually taken place in regard to a good many plants, which have been ranked by each botanist, in his turn, according to his own particular views; at one time in the phænogamous, at another in the cryptogamous, at another in the agamous class. In this way opinions have become divided. One plant has been known to change its place as often as it has been examined. Another, after having been deposited by common assent, in one of the two classes of which we are now speaking, to this hour affords a handle for controversy, because it has not been possible to adduce strict proof of the office of each organ; hence it is, that both in the agamous and in the cryptogamous plants, the appellations of stamen, pistil, anther, germen, pollen, seed, propagulum, &c. &c., have been all applied in their turn to the same part in the same species by different botanists; and that systems have gone on multiplying as fast as any fact in regard to organization, that had escaped preceding observers, has been brought forwards by succeeding ones.

If we give due weight to these circumstances, we shall be convinced, that a different line is to be pursued in reviewing the agamous and cryptogamous department of vegetation, from the one we have had to pursue in considering the phænogamous portion; for here our object cannot be to lay down general positions, and bring into one point of view all that belongs to each system of organs apart, because the forms are extremely various, and their functions, and of consequence their analogies, are more within the scope of conjecture than of demonstration. The method we have to pursue is to confine ourselves to the study of each group by itself, keep separate the facts which are