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lions between the cells of the host and parasite. In a few cases N o e l

B e r n a r d , B u r g e f f , M a g n u s and others have observed the

phenomena analogical to phagocytosis in animals, i. e. the intercellular

digestion o f parasites by the host-cells.

In some cases the physiological reaction manifests itself in immune

varieties by a formation of new tissues under the influence of the parasite,

which isolate the further penetration of fungi (В r u 1 о v).

One of the first theories explaining the nature of the physiological

immunity was the chcmotropic theory of M a a s e e 'which sees the cause

ol immunity in the negative chemotactic influence of the juice of host-cells

on the germ tubes oi fungi. But investigations by F u l t o n and B r o w n

are opposed to the data of M a s s e c ’. Besides, histological investigations

made by G i b s o n , M a r r y a t , S a l m o n ' anti others have shown

that positive chemotactic attraction of germ tubes of fungi is not sufficient

to produce normal growth of fungi on 'the plant.

••

Some authors as L a u r e n t , S e n n , H a g l e r , M a c D o u -

g a 1, R i v e r a believe that high osmotic pressure of the cell sap of host-

cells serves as.a factor of immunity in plants, since it has been stated

that the parasites have always a smaller osmotic pressure than their hosts.

Investigations made by the author (see Tab. 1, p. 64) have not proved

this proposition. In cereals immune as well as susceptible varieties are

often characterised by the same osmotic pressure of cell sap. (The osmotic

pressure was determined by the plasmotic method).

C o m e s recently proposed a new theory of immunity according to

which the quantity of organic acids and tannin in the cell contents has the

decisive role in determining immunity'to disease in plants1). This theory is

supported by Averna-Sacca, Degli Atti, Lo Priore, Scurti, Sica, Kirchner.

Discussing in detail this theory of G о m e s, the author finds it to

be applicable only to a limited circle of phenomena of immunity. The fallo

wing facts are against its general application.

1. Many plants like

Rumex, Oxalis, Berberis, e tc .

characterised by a

great quantity of acid in their cell sap are very susceptible to parasitic fungi.

2. Analysis of many varieties of oats, wheat and roses, different in their

immunity to rusts and mildew (sec Tables 4, 5 ,6 , 7, pp. 78—-81) prove that

there is no definite connection of the quantity of acid in leaves with their

susceptibility to parasitic fungi. Even such varieties as

Avena brevis, A. stri-

gosa, A . bysaniina,

immune to rust, mildew and smut, do not distinguish

themselves in the quantity of acid in leaves in comparison with the very-

susceptible varieties of oats.

Rosa centifolia Crista la

is very susceptible to

rust and mildew notwithstanding the high percentage of acid in its loaves.

Chemical differences undoubtedly occur between immune and sus

ceptible varieties; the analysis of plants may bo of great interest. In the

group of oats and wheat there are considerable differences even in the qtian-

*) Sec

C o n i e s O.

profilassi nella patologia vegetale. 191G.

H. Вавилов*

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