There are some small plants known to botanists as members of the genus _Coleochæte_, which, without being truly parasitic, grow upon certain water-weeds, as lichens grow upon trees. The little plant has the form of an elegant green star, the branching arms of which are divided into cells. Its greenness is due to its chlorophyll, and it undoubtedly has the manufacturing power in full degree, decomposing carbonic acid and setting oxygen free, under the influence of sunlight. But the protoplasmic contents of some of the cells of which the plant is made up occasionally divide, by a method similar to that which effects the division of the contents of the _Peronospora_ spore; and the severed portions are then set free as active monad-like zoospores. Each is oval and is provided at one extremity with two long active cilia. Propelled by these, it swims about for a longer or shorter time, but at length comes to a state of rest and gradually grows into a _Coleochæte_. Moreover, as in the _Peronospora_, conjugation may take place and result in an oospore; the contents of which divide and are set free as monadiform germs. If the whole history of the zoospores of _Peronospora_, and of _Coleochæte_ were unknown, they would undoubtedly be classed among “Monads” with the same right as _Heteromita_; why then may not _Heteromita_ be a plant, even though the cycle of forms through which it passes shows no terms quite so complex as those which occur in _Peronospora_ and _Coleochæte_? And, in fact, there are some green organisms, in every respect characteristically plants, such as _Chlamydomonas_, and the common _Volvox_, or so-called “Globe animalcule,” which run through a cycle of forms of just the same simple character as those of _Heteromita_. The name of _Chlamydomonas_ is applied to certain microscopic green bodies, each of which consists of a protoplasmic central substance invested by a structureless sac. The latter contains cellulose, as in ordinary plants; and the chlorophyll which gives the green colour enables the _Chlamydomonas_ to decompose carbonic acid and fix carbon as they do. Two long cilia protrude through the cell-wall, and effect the rapid locomotion of this “monad,” which, in all respects except its mobility, is characteristically a plant. Under ordinary circumstances, the _Chlamydomonas_ multiplies by simple fission, each splitting into two or into four parts, which separate and become independent organisms. Sometimes, however, the _Chlamydomonas_ divides into eight parts, each of which is provided with four instead of two cilia. These “zoospores” conjugate in pairs, and give rise to quiescent bodies, which multiply by division, and eventually pass into the active state.