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32
W. R. IVIMEy Cook.
In the following pages I intend to give a brief account of the char- acters which distinguish the species from one another, treating the group in a purely systematic method.
Order I Plasmodiophorales.
They are endoparasitic fungi living in the cells of Phanerogams. The vegetative phase consists in the production of a myxamoeba which grows into a multinucleated plasmodium, or amœba, which may be either the pro- duct of a single myxamaba, or the result of coalescence of several amabæ together. Plasmodia may also be produced by fragmentation of plasmodia into uni- or multinucleated schizonts. The nuclei of the plasmodium divide simultaneously by a simple form referred to by most workers as protomitosis. Prior to spore formation the nuclei undergo phase spoken of as the Akaryote stage in which some, or all, stainable chromatin is extruded into the cytoplasm.
Subsequently during the reproductive phase of the life cycle the nuclei divide twice, the first division being the reduction division The two divisions forming and the second a typical homotypic one.
a
An additional
a meiosis strictly similar to that found in higher plants.
one genus. method of reproduction by means of zoosporangia occurs in The spores on germination give rise to swarm spores which are pyriform in shape and have one flagellum. The swarm spore either penetrates a and eventually fresh root or migrates in the tissues of the same tumour, fuses with a mate to produce a myxamœba.
Family I. Plasmodiophoraceœ,
There is only one family which therefore exhibits all the characters of the order. It is divided into six genera which are distinguished by the arrangement of the spores. The following key indicates the chief points of difference between these six genera.
Key to the genera of the Plasmodiophoraceæ.
1. Plasmodium giving rise to spores only.
a. Spores at maturity not arranged in a definite system, but
lying freely in the host cell.
1. Plasmodiophora. Page 33.
b. Spores at maturity arranged in a definite system.
A. Spores arranged in a spore-ball or spore-cake.
1. Spore-balls spherical or ellipsoidal, hollow, containing a single layer of spores; the whole spore-ball being enclosed membrane.
In a common
2. Sorosphaera. Page 34.
2. Spore ball ellipsoidal but not hollow, contain- ing two layers of spores closely applied to one another; the whole spore-ball being en- closed in a common membrane.
3. Sorodiscus. Page 35.
The Hong Kong Naturalist.
11:
The Hong Kong Naturalist Supplement.
No. 1.
Plate 14, figure 1.
Sorosphaera Veronicae, Spores x 320.
Plate 14. figure 2.
Sorosphaera radicale, Spores x 320.
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