. Beihefte zum Botanischen Centralblatt. Botany. 280 Salmon,On Specialization of Parasitism in the Erysiphaceae. Turning again to Table 9, it is plain that on account of the differences here shown the four, or five, Oidia must, adopting current terminology, be called ,biologic forms,' since although agreeing in all morphological characters, they present certain biological peculia- rities. The term ,biologic form' is preferable to that of ,biologic species,' used by some authors, since the latter implies a fixity of characters which may possibly not exist in these cases. One of the most fundame
. Beihefte zum Botanischen Centralblatt. Botany. 280 Salmon,On Specialization of Parasitism in the Erysiphaceae. Turning again to Table 9, it is plain that on account of the differences here shown the four, or five, Oidia must, adopting current terminology, be called ,biologic forms,' since although agreeing in all morphological characters, they present certain biological peculia- rities. The term ,biologic form' is preferable to that of ,biologic species,' used by some authors, since the latter implies a fixity of characters which may possibly not exist in these cases. One of the most fundamental problems that arises in the investigation* of these ,biologic forms' may be presented in the form of the following question: Does an Oidium on passing from one species of host-plant to another acquire at once the same powers of infection as those of the Oidium that has grown for a long time on the latter host- plant? Consideration in detail of four hypothetical cases will help to show more exactly the nature of this problem, and at the same time show the difficulties that arise when we seek for a strict definition of the term ,biologic form.' 1. Let us suppose that an Oidium, Ol, occurs on a certain host-plant a, and that this Oidium proves by experiment to be cap- able of infecting another host-plant b, but is incapable of infecting the plant d. Another Oidium, 02, is found on the host-plant d, and is likewise found capable of infecting b, but not a. These relations may be expressed by the following diagram: — (Fig. 1). Since Ol a is incapable of directly infecting d, and 02 d of directly infecting a, these two Oidia would constitute ,biologic forms,' — according to the definition of the term at present in use. Now, both Ol and 02 infect b. We shall have therefore on b either (1«), two ,biologic forms' with different powers of infection accor- ding as they originated from Ol on a or 02 on b (i. e. 01 b might be able to infect a and not d; similarly with 0 2b); or, (I
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