Page images
PDF
EPUB

But further growth is for a time almost restricted to the lamina until the ratio is increased to ten or fifteen to one. After this stage the stipe begins to grow and soon surpasses the lamina, which seldom exceeds half a meter in length, while the stipe sometimes becomes fifteen or twenty times as long.

A specimen 12 cm. in length (Fig. 32) though only two thirds as long as the one just described, was considerably more advanced. The uppermost quarter of the lamina was entire, as in the last plant, and below the tip were a few serrations like those at the extreme base of the former. Toward the growing point these outgrowths were larger and had become spatulate proliferations about a centimeter long, fringing the basal two thirds of the blade. The stipe had reached a length of 3 cm. Its numerous tubercles were much elongated and frequently dichotomously branched, once or even twice giving the peculiar roughened appearance characteristic of the adult. The proliferations along the lateral edges of the stipe were much more numerous than in the former specimen; some of them were simple laminar appendages; others were inflated into small globular pneumatocysts (Fig. 31, p); on others the stalks were roughened by small tubercles like those of the main stipe. Some of these last, if detached, might easily pass for young plants cut off just above the holdfast.

Though marked changes are yet to occur before the plant becomes mature, they may be understood by a comparison of the adult with this young plant (Fig. 32). The most conspicuous change is of course the great elongation. While this is especially noticeable in the stipe, the lamina likewise grows until it reaches a length of about 50 cm., but its width increases scarcely at all, seldom exceeding 4 cm. The proliferations from this narrow lamina become so numerous that they completely mask the distinction between it and the stipe, and it is only by close inspection that the lamina may be recognized. The growth of the stipe carries the lamina far away from the holdfast, where it is exposed to the severest action of the waves, which lash the plant until the lamina together with the meristem is torn off and there remain simply the stipe and holdfast.

The stipe remains smooth for a few centimeters above the large branching holdfast, this being evidently a persistence of the smooth basal region of the young plant. Some of the lower tubercles, however, disappear, so that the smooth area now extends farther from the base than originally. This portion is terete, but at a length of about a decimeter the stipe becomes flat and strap-like about four times as wide as thick.

In the younger specimens the proliferations from the stipe and lamina are all small and not very numerous. In the adult they enlarge very greatly and increase in numbers so as to become by far the most conspicuous feature of the plant. The increase, both in number and size, is most marked toward the growing point, those at the base generally remaining small and scattered. Farther out along the stipe they are found of all lengths up to about 12 cm. and of various forms, as figured by Ramaley. They stand as thickly as possible along the stipe; in some places by actual count upwards of a hundred were found in a single centimeter of its length. Of these only a few were large and more than half less than a centimeter long. Crowded as they are along the edges of the stipe, they never arise from its faces, which are bare except for the tubercles described above. The air vesicles are formed at frequent intervals, providing sufficient buoyancy to keep the plant floating just beneath the surface with the tips of the proliferations emerging. When mature, they are about 30 mm. long, with an average capacity of about one cubic centimeter. Others of the outgrowths remain permanently small and become sporophylls. The outgrowths on the lamina also increase in size and number, but become neither so large nor so numerous as on the stipe. As noticed by Ramaley, no bladders nor sporophylls develop on the lamina.

Egregia becomes much branched before it is mature. Although Ramaley suggests that the branching may have

[graphic]

Fig. 35. Egregia, base of small plant with the characters of the adult except for the small number of branches, though only a few of those present could be shown, while the others were piled up in a mass to the left of the holdfast, (a) a dwarfed branch with a frilled margin. One fourth natural size.

an appearance similar to Lessonia, it is brought about by a fundamentally different process, as has already been noted by Setchell ('93) and by Reinke (203), who figure several stages in the development of a branch. Some of the earlier proliferations, as stated above, soon develop roughenings on their stalks like those of the main stipe and take on the appearance of younger specimens of the species (Fig. 31, b). This is the first external indication of an important difference in the constitution of these outgrowths from the ordinary proliferations. For in them has become differentiated a meristem independent of that of the primary branch. They develop exactly as did the main axis and soon become indistinguishable from it except in the manner of attachment to the holdfast, possessing all the structures which have been described for a primary branch including other branches which in turn go through the same process. After several such branches have been formed there is a modification of the process. The laminæ are dwarfed, while their margins become conspicuously puckered and ruffled (Fig. 35, a). Sometimes the ruffles are so pronounced as to completely enfold the meristem. In such a branch proliferations from the lamina appear very late, but the ruffle gives it a similar aspect. The dwarfed condition of the laminæ persists until the stipes become several centimeters in length, when the usual relations of stipe and lamina become manifest. Though roughening may appear on other parts of the plant, the development of meristematic proliferations is confined to the basal portion; branches do not develop at a distance much exceeding 20 cm. from the holdfast. Around the base of any old plant there is always a large number of short branches in all stages of development, but there are not often more than a dozen long branches at any one time. The general appearance of the numerous dwarf branches suggests that they may not have a rapid development like the first branches, but rather grow very slowly or lie dormant for a time like the dormant buds of trees.

This method of branching is peculiar to Egregia and, as

[graphic]

Fig. 36. Egregia growing in a thick bed of kelp in which are prominent Alaria (with a midrib) and Hedophyllum (in foreground, especially at right).

far as the writer knows, nothing like it occurs in other kelps save in Thallasiophyllum. It is a matter of great interest from several points of view. Morphologically it gives the best reason for considering Egregia the highest of the Alariatæ', although that position would probably be accorded it without question because of the differentiation of the ordinary proliferations alone. The other members of this subfamily produce outgrowths which function as sporophylls, and in some of them, e. g., Eisenia, these become the main photosynthetic areas of the plant. The development of meristems in such outgrowths, leading to the formation of branches, is the next step towards greater complexity and the logical summit of the Alaria series. But its greatest interest is from the ecological point of view. The extreme length of the stipe pushes the growing point far out, where it is lashed severely by the waves and frequently destroyed. Were the plant dependent on this for its continued healthy existence, as in Laminaria, it might easily be killed or at least handicapped for a considerable part of the time by the loss of the blade until a new one could be regenerated, as in many species of Laminaria. But, should the older branches be injured, these basal branches may develop at any time. By their

« PreviousContinue »