CHAPTER XVII._Elementary Botany

COLEOCHÆTE.

=322.= Among the green algæ coleochæte is one of the most

interesting. Several species are known in this country. One of these at

least should be examined if it is possible to obtain it. It occurs in

the water of fresh lakes and ponds, attached to aquatic plants.

[Illustration: Fig. 155. Stem of aquatic plant showing coleochæte,

natural size.]

[Illustration: Fig. 156. Thallus of Coleochæte scutata.]

=323. The shield-shaped coleochæte.=—This plant (C. scutata) is

in the form of a flattened, circular, green plate, as shown in fig.

156. It is attached near the center on one side to rushes and other

plants, and has been found quite abundantly for several years in the

waters of Cayuga Lake at its southern extremity. As will be seen it

consists of a single layer of green cells which radiate from the center

in branched rows to the outside, the cells lying so close together as

to form a continuous plate. The plant started its growth from a single

cell at the central point, and grew at the margin in all directions.

Sometimes they are quite irregular in outline, when they lie quite

closely side by side and interfere with one another by pressure. If the

surface is examined carefully there will be found long hairs, the base

of which is enclosed in a narrow sheath. It is from this character that

the genus takes its name of coleochæte (sheathed hair).

[Illustration: Fig. 157. Portion of thallus of Coleochæte scutata,

showing empty cells from which zoogonidia have escaped, one from each

cell; zoogonidia at the left. (After Pringsheim.)]

[Illustration: Fig. 158. Portion of thallus of Coleochæte scutata,

showing four antheridia formed from one thallus cell; a single

spermatozoid at the right. (After Pringsheim.)]

=324. Fruiting stage of coleochæte.=—It is possible at some

seasons of the year to find rounded masses of cells situated near the

margin of this green disk. These have developed from a fertilized egg

which remained attached to the plant, and probably by this time the

parent plant has lost its color.

=325. Zoospore stage.=—This mass of tissue does not develop

directly into the circular green disk, but each of the cells forms a

zoospore. Here then, as in œdogonium, we have another stage of the

plant interpolated between the fertilized egg and that stage of the

plant which bears the gametes. But in coleochæte we have a distinct

advance in this stage upon what is present in œdogonium, for in

coleochæte the fertilized egg develops first into a several-celled mass

of tissue before the zoospores are formed, while in œdogonium only four

zoospores are formed directly from the egg.

=326. Asexual reproduction.=—In asexual reproduction any of the

green cells on the plant may form zoogonida. The contents of a cell

round off and form a single zoogonidium which has two cilia at the

smaller end of the oval body, fig. 157. After swimming around for a

time they come to rest, germinate, and produce another plant.

=327. Sexual reproduction.—Oogonium.=—The oogonium is formed by

the enlargement of a cell at the end of one of the threads, and then

the end of the cell elongates into a slender tube which opens at the

end to form a channel through which the spermatozoid may pass down to

the egg. The egg is formed of the contents of the cell (fig. 159).

Several oogonia are formed on one plant, and in such a plant as C.

scutata they are formed in a ring near the margin of the disk.

[Illustration: Fig. 159. Coleochæte soluta; at left branch bearing

oogonium (_oog_); antheridia (_ant_); egg in oogonium and surrounded

by enveloping threads; at center three antheridia open, and one

spermatozoid; at right sporocarp, mature egg inside sporocarp wall.]

[Illustration: Fig. 160. Two sporocarps still surrounded by thallus.

Thallus finally decays and sets sporocarp free.]

[Illustration: Fig. 161. Sporocarp ruptured by growth of egg to form

cell mass. Cells of this sporophyte forming zoospores.

Figs. 160, 161. C. scutata.]

=328. Antheridia.=—In C. scutata certain of the cells of the

plant divide into four smaller cells, and each one of these becomes

an antheridium. In C. soluta the antheridia grow out from the end of

terminal cells in the form of short flasks, sometimes four in number or

less (fig. 159). A single spermatozoid is formed from the contents. It

is oval and possesses two long cilia. After swimming around it passes

down the tube of the oogonium and fertilizes the egg.

=329. Sporocarp.=—After the egg is fertilized the cells of the

threads near the egg grow up around it and form a firm covering one

cell in thickness. This envelope becomes brown and hard, and serves

to protect the egg. This is the “fruit” of the coleochæte, and is

sometimes called a sporocarp (spore-fruit). The development of the cell

mass and the zoospores from the egg has been described above.

Some of the species of coleochæte consist of branched threads, while

others form circular cushions several layers in thickness. These forms

together with the form of our plant C. scutata make an interesting

series of transitional forms from filamentous structures to an expanded

plant body formed of a mass of cells.

=330. COMPARATIVE TABLE FOR SPIROGYRA, VAUCHERIA, ŒDOGONIUM,

COLEOCHÆTE.=

-----------+--------------------------------------------------------

| GAMETOPHYTE. (Bears the sexual organs and gonidia.)

-----------+-----------+---------+------------+---------------------

|Vegetative | Growth. |Multipli- | Sexual Reproduction.

| Phase. | | cation.+---------------------

| | | | Sexual Organs.

-----------+-----------+---------+------------+---------------------

Spirogyra. |Simple |All |By breaking | Undifferentiated.

|threads of |cells |up of | Any cell of thread.

|cylindrical|divide |threads. | Conjugate by tube.

|cells. |and | |

| |grow. | |

-----------+-----------+---------+------------+---------------------

|Branched |Limited |By | Differentiated.

| | | | opens

-----------+-----------+---------+------------+----------+----------

|Simple |Limited |By oval | Differentiated.

-----------+-----------+---------+------------+----------+----------

Coleochæte.|Branched |Terminal |By | Differentiated.

-----------+-----------+---------+------------+----------+----------

-----------+------------------------+-------------+-----------

| GAMETOPHYTE. | |

| (Bears the sexual | |

| organs and gonidia.) | SPOROPHYTE |How Veg.

-----------+------------------------+ Bears |Phase of

| Sexual Reproduction. | spores |Gametophyte

+------------------------+-------------+ is

| Gametes. | Fruit. |Developed.

-----------+------------------------+-------------+-----------

Spirogyra. | Undifferentiated. | Zygospore |Develops

| Entire contents of | Rests. |veg. phase

| conjugating cell. | |directly.

-----------+------------------------+-------------+-----------

| Differentiated. | Egg (or |Develops

|Small | Large | oospore). |veg. phase

Vaucheria. |two-ciliated | egg | Rests. |directly.

|spermatozoids.| cell. | |

-----------+--------------+---------+-------------+-----------

| Differentiated. | Egg (or |Divides

|Oval | Large | oospore). |into

Œdogonium. |spermatozoids | egg | Rests. |four cells;

|with | cell. | |each

|crown of | | |forms

|cilia. | | |zoospore

|Two from each | | |which

|antheridium. | | |develops

| | | |veg. phase

| | | |again.

-----------+--------------+---------+-------------+-----------

Coleochæte.| Differentiated. |Egg |Each forms

|Oval, | Large |(surrounded |a zoospore.

|biciliate | egg |by |Zoospore

|spermatozoid, | cell. |wall from |develops

|one from each | |gametophyte).|veg. phase

|antheridium. | |Rests. |again.

| | |Divides |

| | |and |

| | |grows to |

| | |form a mass |

| | |of cells. |

-----------+--------------+---------+-------------+-----------