A RACE OF FLOWERLESS PLANTS— V. 



HOW FUNGI INJURE PLANTS. 



N EARLIER times the opponents 

 of the "germ theory" of dis- 

 ease in plants were commonly 

 ready to admit the presence of 

 a fungus — in fact, to determine 

 its presence was a simple mat- 

 ter of observation ; but they contended that it 

 merely accompanied or followed and did not cause 

 the disease. Their position was the logical one to 

 take until the opposite could be clearly shown to be 

 true, and to show that the fungus causes the dis- 

 ease, makes it necessary or at least very important, 

 to show in what the noxious effect consists. 



In some cases, as in corn smut, the fungus causes in- 

 juries of such a character that they are evident to all. 

 In others, the character of the injury is much more 

 difficult to determine. There are several kinds of^in- 

 juries which a fungus may produce in the plant it at- 

 tacks, and often several may be caused at the same time. 



The one first to be mentioned is the most universal 

 and comes necessarily on account of the essential na- 

 ture of fungi. Fungi have no green coloring matter, 

 and for this reason cannot supply themselves with food 

 from the earth and air as green plants do ; they are 

 compelled to take their food from some ready-prepared 

 organic matter, /. from some living plant or animal 

 or from their dead remains. If they take it from the 

 latter, they are not true parasites, but are called sapro- 

 phytes. Parasite fungi are those which take their food 

 from their "host," that is their supporting plant (or 

 animal) while it is yet alive. Thus an infected plant is 

 deprived of the food which 

 it has supplied for its own 

 use ; the fungus thrives, 

 and the host plant suffers 

 more or less from starva- 

 tion. Many fungi, such as is,^ 

 grape mildew and its al- 

 lies, have special organs 

 (suckers) (Fig. i) for ab- 

 sorbing nutritive material 

 from the cells of plants, 

 and the cell walls are also 

 punctured, ruptured or 

 otherwise injured. The 

 suckers and mycelium f-^ 

 which enter a cell some- 

 times kill it and sometimes 

 allow it to grow on with 

 reduced vitality, a con- 

 tinual prey. Fig. i. Suckers in Cells. 



Fig. 2. Spot Dise.\se of Curr.^nts. 



If the green parts are the ones attacked, as most fre- 

 quently they are, the power of the plant to provide 

 fresh food for itself is also impaired. 



The light may be kept from the leaf to some extent, 

 as when the 

 fungus 

 forms a 

 layer over 

 the surface. 



S o m e - 

 times leaves 

 are covered 

 with the 

 e.xudati o n s 

 of insects 

 ( " honey- 

 dew ") and 

 a black fun- 

 g u s grows 

 on this, 

 covering the 

 surface o f 

 the leaf. 



Also, the 

 active pow- 

 er o f the 



leaf-green is less on account of the weaker vitality to 

 provide food for itself. 



Sometimes portions of the leaf are killed and appear 

 as brown dry spots, as in the spot disease of the garden 

 currant (Fig. 2) and many other plants ; or portions 

 even die and fall out, leaving holes. When the dam- 

 age is less severe, spots of various colors are seen, more 

 or less characteristic of some special disease. Fig. 3 

 shows spots on mignonette leaf. Still other fungi 

 cause the entire leaf, or a considerable portion of it, to 

 become withered and dry, and it is usually the free end 

 of the leaf and the leaves nearest the base of the stem 

 that suffer most, as in rusted cereals. Certain plants, 

 as Canadian thistle, attacked by their peculiar rust, 

 wilt in hot sunshine, afterwards reviving ; but healthy 

 thistles do not wilt. 



Another common effect is to change the size of a 

 plant, to make it grow either less or more rapidly 

 than a healthy plant. 

 We might easily expect a 

 diseased plant to grow less 

 rapidly, but strangely enough 

 the reverse is very common. I'lG- 3- Spot Dise.^se of 



. I,- u i ■ Mignonette. 



A spurge on which a certain 



common cluster-cup is growing, and a blackberry in- 

 fested with orange rust, have smaller leaves but taller 



