Page:EB1911 - Volume 19.djvu/124

 The condition of the capillitium is very various. In the Calcarineae the lime may be generally distributed through it (fig. 11), or aggregated at the nodes of the network in “lime-knots” (figs. 12 and 14) or it may be absent from the capillitium altogether. The capillitium attains its highest development in the Calonemineae in which the threads, distinct (in which case they are known as elaters, figs. 9 and 10) or united into a network (fig. 20), present regular thickenings in the form of spiral bands or transverse bars. These threads, altering their shape with varying states of moisture, are efficient agents in distributing the spores. In another group, the Anemineae, the capillitium is absent altogether.

The Didymiaceae are characterized by the fact that the lime, though present in a granular form in the plasmodium, is deposited on the sporangium-wall in the form of crystals, either in radiating groups (fig. 15) or in disks (fig. 16).

In most Endosporeae the sporangia are separate symmetrical bodies, but in many genera a form of fructification occurs in which the spores are produced in masses of more or less irregular outline, retaining in extreme cases much of the diffuse character of the plasmodium. With the spores they contain capillitium, but there are no traces of sporangial walls to be found in their interior. They are known as plasmodiocarps (fig. 22). They are characteristic of certain species, but in others they may be formed side by side with separate sporangia from the same plasmodium. There is indeed no sharp line to be drawn between sporangia and plasmodiocarps. On the other hand, the crowded condition of the sporangia of some species forms a transition to the large compound fructifications known as aethalia (fig. 21). These, either in their young stages or up to maturity, retain some evidence of their formation by a coalescence of sporangia, and in addition to the capillitium they are generally penetrated by the remains of the walls of the sporangia which have thus united.

Exosporeae.

It will be convenient to begin our survey of the life-history of Ceratiomyxa, the single representative of the Exosporeae, at the stage at which the plasmodium emerges from the rotten wood in which it has fed. At this stage it has been observed to spread as a film over a slide, and to exhibit the network of channels and rhythmic flow of the protoplasm in a manner precisely similar to that seen in the Endosporeae (20, p. 10). It soon, however, draws together into compact masses, from the surface of which finger-like or antler-like lobes grow upwards. Here too the secretion of a transparent mucoid substance occurs, which is at first penetrated by the anastomosing strands of the protoplasm, but gradually the latter tends more and more to form a reticular and ultimately a nearly continuous superficial investment, covering the mucoid material. The latter eventually dries and forms the exceedingly delicate support of the spores or sporophore (fig. 23, 𝑎).

The investing protoplasm, with its nuclei, having become arranged in an even layer, undergoes cleavage and thus forms a pavement-like layer of protoplasmic masses, each occupied by a single nucleus

(fig. 23, 𝑏). Each of these masses now grows out perpendicularly to the surface of the sporophore. As it does so an envelope is secreted, which, closing in about the base forms a slender stalk. The minute mass, borne on the stalk, becomes the ellipsoid spore, surrounded by the spore-wall. In this manner the whole of the protoplasmic substance of the plasmodium is converted into spores, borne on supporting structures (stalks and sporophores), which are formed by secretion of the protoplasm.

In the course of the development of which the external features have now been traced nuclear changes occur of which accounts have been given by Jahn (14) and by Olive (24 and 25). Jahn has shown that prior to the cleavage of the protoplasm a mitotic division of the nuclei takes place, the daughter nuclei of which are those occupying the protoplasmic masses seen in fig. 23 𝑏. After the spore has risen on its stalk two further mitotic divisions occur in rapid succession, and the four-nucleated condition characteristic of the spore of Ceratiomyxa, is thus attained. The spores, on being brought into water, soon hatch (fig. 23, 𝑑), and the four nuclei contained in them undergo a mitotic division. Meanwhile the protoplasm divides, at first into four, then into eight masses, and the latter acquire flagella, although for some time remaining connected with their fellows (fig. 23, 𝑒-ℎ). On separating each is a free zoospore.

From observation of cultivations of zoospores the impression is that here, as in the Endosporeae, they multiply by binary division, though no exact observations of the process have been recorded. The zoospores lose their flagella and become amoebulae, but the fusion of the latter to form plasmodia has not been directly observed in Ceratiomyxa, although from analogy with the Endosporeae it can hardly be doubted that such fusions occur.

Sorophora.

The Sorophora of Zopf (Acrasiae of Van Tieghem) are a group of microscopic organisms inhabiting the dung of herbivorous animals and other decaying vegetable matter. As Pinoy (26) has shown, the presence of a particular species of bacteria with the spores is necessary for their hatching and as the essential food of the amoebulae which emerge from them. There is no flagellate stage, and it is in the form of amoebulae, multiplying by fission, that the vegetative stage of the life-history is passed. At the end of this stage numbers of amoebulae draw together to form a “pseudo-plasmodium.” This appears to be merely an aggregation of amoebulae prior to spore formation. The outlines of the individual amoebulae are maintained, and there is no fusion between them, as in the formation of the plasmodium of the Euplasmodida.

In some genera certain of the amoebulae constituting the pseudo-plasmodium are modified into a stalk (simple in Guttulina and Dictyostelium, branched in Polysphondylium, fig. 24, 𝑑), along which the other units creep to encyst, and become spores at the end or ends of the stalk. In other cases (Copromyxa, fig. 24, 𝑎 and 𝑏) the pseudo-plasmodium is transformed into a mass of encysted spores without the differentiation of supporting structures.

It is not impossible that the Myxobacteriaceae of Thaxter may, as that author suggests, be allied to the Sorophora (30).

Review of the Life-Histories of the Mycetozoa.—The data for a comparison of the life-history of the Mycetozoa with those of other Protozoa in respect of nuclear changes are at present incomplete.