34 Chapter II 



cells of the two belligerent plants^ an activity dependent upon the 

 irritability of their protoplasm. 



The example we have just studied may serve as a type for our 

 examination of the phenomena of immunity in the vegetable kingdom. 

 The crux is above all to prevent the access of the parasites to the 

 vital parts of the plant by opposing to them membranes as resistant 

 as possible. Consequently the majority of plants, directly the smallest 

 lesion is produced, react by an abundant cell-proliferation and by the 

 suberisation of the outer layers. The cell-membranes of the latter 

 thicken, the cellulose is transformed into suberin ; a layer of cork 

 not very permeable to fluids and gases being thus formed. By 

 suberisation the plant reacts against grosser lesions, incisions or 

 burns, as well as against the decay set up by micro-organisms. 



Massart\ in an extremely interesting memoir, has brought 

 together the known data concerning cicatrisation in plants and has 

 demonstrated the fact that it is a very variable process. In many 

 leaves after being damaged there is no attempt to react by foiming 

 cicatricial tissue. Many aquatic and marsh plants react but feebly. 

 Their tissues die and turn brown, the plants failing to defend them- 

 selves by cicatrices, probably owing to the ease with which the lost 

 parts can be replaced. When, however, in the same plants, there is 

 produced a lesion of parts which are of great importance for the 

 preservation of the integi'ity of the individual or a lesion of the organs 

 which enable the plant to continue its existence through the winter, 

 cicatrisation of the wounds takes place rapidly. 



The old or adult parts in most cases react differently from the 

 young parts. Thus, the young leaves of Clida (the example selected 

 by Massart) react to traumatism very promptly and form a genuine 

 [37] callus which makes good the injury, but the adult leaves merely pro- 

 duce a layer of cork in the immediate neighbourhood of the lesion. 



The essential mechanism of cicatrisation has not yet been satisfac- 

 torily analysed, but it is evident, when all is said and done, that it is 

 directed by the irritability of the living protoplasm of the vegetable 

 cells. 



Many plants protect their wounds with a kind of dressing, using 

 for that purpose juices which harden on exposure to the air. Some- 

 times these juices, e.g. latex, are preformed in the plant and are as it 

 were always ready for use ; at other times they may be formed only 



^ " La cicatrisation chez les v^getaux," Mem. common, de VAcad. roy. de Belgique, 

 Bruxelles, 189S, t. lvil 



