Page images
PDF
EPUB
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed]

Figs. 18-23. Lessoniopsis. Four fifths natural size.

Fig. 18. Young plant at about the same stage as Fig. 15 to which is attached another intermediate between it and that shown in Fig. 4.

Fig. 19. Plant grown in heavy surf, holdfast very large; plant dwarfed, indentation for first branch already appearing in the transition region.

Fig. 20. Plant from quiet water grown to an unusually large size, with no indication of branching, midrib just forming.

Fig. 21. Similar to the last except for the beginning of the perforation. FIG. 22. Perforation complete.

Fig. 23. Primary branching complete, perforations formed for second branches, inner side of new laminse beginning to form from the divided midrib.

[ocr errors]

to which Lessoniopsis belongs, is characterized by the repeated splitting of the original unbranched lamina till the plant comes to have a cluster of many leaves. The method of this branching is peculiar to the kelps. Instead of forking at the tip or sending out a new shoot as a lateral proliferation, the branching begins in the trans ition region between the stipe and the lamina and extends upward until it reaches the tip of the lamina, thus splitting it, while the stipe is divided, to a greater or less extent in the different genera, by the downward extension of the same process. This method of branching is the necessary consequence of the position of the meristem, which is situated at the junction of lamina and stipe, so that all new growth is intercalated between the older portions of both. It is obvious, therefore, that any new structure, such as a branch, must originate in this region of growth.

In Lessoniopsis the first indication of branching appears in a slight depression in the midrib on each side of the lamina at the transition region. These depressions or pits enlarge and deepen until they meet and form a perforation almost exactly at the base of the lamina. It will be readily seen that if the split extended uniformly upward through the midrib, it would result in two unsymmetrical falcate laminæ each with a rib along its inner side. This, however, is not usually the case, for new tissue forms between the divisions of the midrib and soon duplicates on the inner side the outer edge of the lamina (Fig. 24, a). Thus each of the new laminæ is approximately symmetrical with respect to its midrib. In the stipe the branching is carried far enough to involve the whole of the meristem, so that future lengthening is almost completely confined to the stipes of the branches.

Before the new laminæ have completely separated there usually begins to appear at the base of each, the second split, which is carried to its completion in the same manner as the first. Thus branching continues again and again so long as the plant lives. Since all the splitting is dichotomous, the result should be a flat fan-shaped plant,

[graphic][subsumed]

Figs. 24-26. Older Lessoniopsis. About one sixth natural size.

Fig. 24. Plant several times branched ; at a the process of the formation of the inner side of the divided lamina from the midrib is clearly indicated.

Fig. 25. Plant with about 25 lamine, original dichotomy plainly shown.

Fig. 26. Plant still undersized but with the characters of the adult; one branch is lifted out by a background to show the sporophylls (8) and their relation to the ordinary laminæ, which show the beginnings of division at their bases as in the younger forms.

Cl

and sometimes this form is attained even in very old plants, especially those growing in the quieter places, but usually the stipes twist more or less and spread out in all directions, giving the plant a tree-like aspect.

There is no change in this habit of growth until the plant has attained a considerable age. But long before it reaches its full size there appears another kind of lamina among the narrow ones with midribs. These lack the midribs and are much wider, with conspicuously rounded or subcordate bases. The ribbed laminæ are always sterile, but these wider ones become sporophylls. Consequently after their sporangia are discharged they slough off and disappear, leaving for a time scars on the stipe. The origin of these sporophylls is evidently different from that of the ordinary laminæ. Since very few new sporophylls are developed during the summer, at

least at Port Renfrew, it seems probable that their production is a seasonal phenomenon taking place only for a limited period before the fruiting season. However they are formed, they do not reach their full size at first. The youngest are always shorter and narrower than the older and entirely lack the characteristic base. Some of the smallest remind one of the young sporophylls of Pterygophora and have the appearance of being outgrowths from the meristem as in that species, but the writer does not feel sure that they are normal. Further information on the origin of the sporophylls will be very welcome because of its importance in determining the relationships of this plant to the other genera of kelps.

At length, by branching and production of sporophylls a plant is formed with several hundred laminæ, in extreme cases reaching lengths of a meter, while the whole plant is often two meters long. The stipe at the base becomes 10-20 cm. in thickness and is marked with many annual rings of growth. The holdfast clings so tenaciously to the rocks that it will support a man's weight. On a flat bottom the plants stand upright, but they hang down when growing on an overhanging cliff, as in the photograph (Fig. 27). As in all water plants, their only way of maintaining themselves in the strong currents in which they live is by bending before them. Accordingly, rigidity is developed only in very large basal portions of the stipe, while the terminal branches have not sufficient stiffness to support the plant when out of the water. Lessoniopsis thrives only in places where the surf is very heavy and is there found along with Postelsia, the sea palm, the most typical of all the cumaphytes, but it does not withstand drying so well as that plant and consequently grows at a considerably lower level.

C. Egregia To one acquainted with the kelps only through the more widely distributed genera such as Laminaria and Alaria, Egregia must always be the most interesting of the family. Algologists agree in assigning to this plant the high

[graphic]

Fig. 27. Lessoniopsis (hanging) and Postelsia (upright) growing on an overhanging shelf exposed to the heaviest surf. Lessoniopsis is about two meters long and Postelsia, one half meter.

est place among the kelps as being the most specialized of them all. It is a genus of the western coast, represented by two species, one northern, the other southern. Both are extremely variable and in their many forms and intergradations present to the taxonomist a problem of more than usual difficulty. Some features of the morphology of the northern species, Egregia menziesië, have been presented in a paper by Ramaley (03), illustrated with some excellent figures of adult and middle-aged plants, while Reinke (03) has also given figures and a brief description of somewhat younger plants. The development of this species which grows abundantly at the Minnesota Seaside Station, will be worth considering in detail in connection with the other kelps discussed above because of its greater complexity.

Egregia, like Nereocystis, has an extremely long stipe; indeed, in proportion to its lamina its stipe is much longer, but its character is totally different from that of Nereocystis. In the latter plant the stipe stretches from the holdfast, frequently attached to a depth of twenty or

« PreviousContinue »