Plants; a text-book of botany by Coulter John Merle 1851-1928

Author:Coulter, John Merle, 1851-1928
Language: eng
Format: epub
Tags: Botany
Publisher: New York, D. Appleton and company
Published: 1900-03-25T05:00:00+00:00

Pia IS Tvo ConJDgsle fa A (Xotifftollat, showinj madon at lygcite Iti cnn

by walls, or they are ccenocytie, as in the Siphon forma. The characteristic asexual spores are zoospores, but these may be wanting, as in the Conjugate forms. In addition to asexual reproduction, both isogamy and heterogamy are developed, and both zygotes and oospores are resting spores.

The Green Algte are of special interest in connection with the evolution of higher plants, which are supposed to have been derived from them.

3. Pnx,OPTiYGEJE.{Iirown Algm)

28. General oharatiten. —The Blue-green Algie and the

Green Algse are characteristic of fresh water, but the Brown AlgEB, or " kelps," are slmost all marine, being very charac-

terifltic coast forma. All of them are anchored by holdfastB, which are sometimes highly developed root-like atructurea; and the yellow, brown, or olive-green floating bodies are buoyed in t aid of floats or air-bia very conspicuous. The developed in the colder of the "wrack," "tang The group is well adt live espoaed to waves ; renta with its strong h air-bladdera, and tough bodies. It is what is k a gpecialued grovp —tha which has become high] ized for certain specit tions. It is not our purpose to consider such a specialized group in any detail, as it does not usually help to explain the atructures of higher groups.

39. The plant body. —There is very great diversity in the structure of the plant body. Some of them, as Ectocar-

,-r- , (n ci ^'"- 18. A btowii alga IBctocanmii, Bhowing a

pus (Fig. 18),are fil bodjcor.»l«ii.go(..inipl»fll«ni«nt»'hlchimt.

amentous forms, like *"" *

the Confervas among ^^l

the Green Algffi, but

others are very much more complex. The thallus of Lam-

inaria is like a huge floating leaf, frequently nine to ten

feet long, whose stalk develops root-like holdfasts (Fig. 18a). The largest body is developed by an Antarctic Laminaria form, which riseB to the surface from a sloping bottom with a floating thallus six hundred to nine hundred feet long. Other forma rise from the sea bottom like trees, with thick trunks, numerous branches, and leaf-like appendages.

The common Fucus, or " rock weed," is ribbon-form and constantly branches by forking at the tip (Fig. 19). This method of branching is called dickolainous, as distinct from that in which branches are put out from the sides of the axis (moHopodial). The swollen air-bladders distributed throughout tlie body are very conapicuons.

The most differentiated thallus is that of Surgasaum (Fig. 20), or " gulf weed," in which there are slender branching stem-like axes bearing lateral members of various kinds, some of them like ordinary foliage leaves; others are floats or air-bladders, which sometimes

resemble clusters of ben;??; and other branches bear the sex organs. All of these structures are but different regions of a branching thallus. Sargassum forms are often torn from their anchorage by the waves and carried away from the coast by currents, collecting in the great sea eddies

PLANT STRUCTURES

produced by oceanic cmreuts and forming the eo-called 'Sargasso seaa," as that of the Korth Atlantic.

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