Page:The Botany of the Antarctic Voyage.djvu/496

460 different textures, all, however, very gelatinous, and odifications of the three layers forming the leaf, there are 1st, the superficial tissue (or cortex) consisting of small cells, closely packed and full of chromule, gradually opening out into, 2nd, an intermediate tissue of much larger cells more loosely placed, with little or no contained chromule, separated by much gelatine ; and 3rd, an elliptical core placed in the long axis of the petiole, composed of still smaller cells, separated by broader masses of gelatine, which latter is permeated by canals, full, as are the small cells, of chromide.

2. Each ramulus, from which proceed the two petioles, whose structure we have just described, presents no very important difference from them ; the core no longer stretches across it, however, but the whole petiole within the superficial portion is augmented by a newly developed though indistinct zone of cellular tissue, thus deposited between the superficial (or cortical) and intermediate tissue. At this period the cortex is somewhat broader, and the intermediate tissue has become, through the absorption of the gelatine, much more conspicuous ; the cells being larger and the spaces between them narrower; little or no change is perceptible in the core itself.

3. The branch is very materially different from either of the above, for what was hitherto the petiole is now enclosed (all but its cortex) in a very broad zone of cellular tissue, whose cells are large and thin towards the old tissue, elongated and of a different shape, so as to show the line of separation between the two periods of growth (see B 1, of the plate Lessonia).

From this time forward the normal mode of growth followed by the stem exhibits an additional layer or zone of cellular tissue for every subdivison of the frond, (shown at A 1, where six are interposed between the cortex and core). It is not probable, however, that this numerical relation can be always evident, or that the number of subdivisions of the frond will indicate the rings of growth in a large stem. This uncertainty arises from the branches being frequently broken off; added to which, the growth of the sea-weed is very rapid, and there being no period of rest, irregular zones may be expected, or their absence from those branches of the plant whose leaves are injured.

In their anatomy the stems of L.fuscescens and L. nigrescent do not differ much from that of this species, except that the air-cells are copious in the stems of the former, and much rarer in the latter ; in which also the cortical substance is much broader.

In the elegant Lessonia Sinclairi, Harv. MSS., from California, the stipes (which bears but a solitary linear frond) is terete, and in the specimen we examined, contains a central core, reaching half-way across the diameter. There are apparently two rings of tissue beneath the cortex, separated by a zone of very large cells (air-cells?); whence it is difficult to account for the stem being terete, for the frond is plane, and the core three times longer than broad. Nor is it easy to explain the origin of the two zones surrounding the core ; if they really be successively deposited, it is possible that the frond is two years old; if not, that the large cells are air-cells, and do not indicate a line of separation between two successive deposits.

I have stated the growth of the Lessonia to be very rapid ; this is proved by the zones of a five-ringed stem being progressively broader towards the circumference. The probability, too, of one being added for every time the laminae divide, and the fact that the process of subdivision is continued in geometrical progression, all favour the opinion that these Algce attain their enormous bulk in a very few mouths. The vast masses washed up on the outer eastern shores of the East Falkland Island, and the rapidity with which they decay, are additional proofs of a singularly rapid development.

The analogy between the mode of growth exhibited by this genus and an Exogenous tree, is, though incomplete, very obvious; both increase by layers deposited outside one another, within a cortical substance, and both contain an axis of tissue different from that forming the greater part of the trunk : here, however, there are no traces of medullary rays. We conclude this subject with the observation, that the periodical increment of the trunk being dependent on, or coincident with, the formation of the laminae, these appear to perform the office of the leaves in the higher order of plants ; and that the Lessonia is also in this respect analogous to an Exogenous plant,