158 CROWN-GALL OF PLANTS. 



species. Certainly there are not as many species as there are host 

 plants, and the ease with which in many cases cross-inoculations 

 take place points rather to one collective species. The monotonous 

 morphology also points in the same direction, but the evidence is 

 not all in. In this connection the reader is advised to make com 

 parisons with the literature on root tubercles of Leguminosae. 



The differences we have observed may be noted by consulting the 

 tables and these seem to be real differences, e. g., slight differences 

 in color or amount or texture of growth, ability or inability to grow 

 in Cohn's solution, reactions in litmus milk, toleration of acids, etc. 

 The difficulty is we do not know exactly what these things mean in 

 the microbian economy, nor what weight to give them as differential 

 characters. All told, the points of resemblance or agreement so far 

 as we have studied the subject are much more numerous and salient 

 than the differences, and for the present at least we prefer to leave 

 the group undivided, merely indicating the various cultures for pur- 

 poses of convenience by the name of the plant from which derived, 

 as daisy, peach, poplar, etc. 



Certain very practical questions arise here, viz, woukl it be advis- 

 able in nursery and orchard practice to follow one galled crop by 

 another crop subject to galls, e. g., peaches by apples or pears, rasp- 

 berries by grapes or quinces, or roses by almonds? Admitting 

 frankly that we do not yet know the extent of artificial cross-inocula- 

 bility, much less that of natural cross-inoculability, and that many 

 more observations and experiments need to be made before we can 

 be quite certain in particular cases to what extent the galls are 

 naturally cross-inoculable, the grower who reads carefully the evi- 

 dence detailed in this bulletin will probably hesitate to take the risk. 



LOCAL REACTION OF THE INOCULATED PLANT. 

 YOUNG VERSUS OLD TISSUES. 



In general old and hard, slow-growing tissues are not favorable to 

 the development of this disease, whatever the host species concerned. 

 Inoculations into such tissues frequently failed. Inoculations into 

 dormant tissues usually failed, 3ven though such tissues began to 

 grow in the course of a few weeks. Mature tissues are not suited for 

 inoculation experiments. 



The most uniform success was had when the inoculations were 

 made into young and rapidly growing parts. In such cases it is often 

 possible to obtain 100 per cent of infections (see Table II). Appar- 

 ently it is sufficient to introduce the bacteria into any actively divid- 

 ing tissues of root or shoot — cambium, xylem, phloem, bark, pith, or 

 mesophyll. Whether the structure of the tumor tissue in such cases 



213 



