DISEASES OF PLANTS. 139 



them." The cells, which I have previously described, are ready- 

 to make good any loss of tissue. lu some cases the repair forms 

 an excresence, and this we term a gall. You have, no doubt, 

 observed them upon the leaves of many species of willow and oak. 

 Some of these galls in "Eastern countries are of a rich purple 

 color, and are varnished over with a soft substance of the consis- 

 tence of honey. They shine with a brilliant lustre in the sun, 

 and appear like a tempting fruit, but when chewed they have an 

 intensely bitter taste. By some botanists they are supposed to be 

 the Apples of Sodom." The ordinary spruce is liable to be 

 attacked by an insect which produces a curious disease in the 

 plant. " The disease consists of an altei'ation in the color and form 

 of the leaves, which become aggregated together in the shape of 

 cone-like excresences. A naturalist who has carefully investigated 

 the subject, says that " the original matriarch lives outside the 

 spruce gall, remaining all winter in a dwarf state at the root of a \ 

 bud. As soon as the bud swells, she revives likewise, and 

 speedily becoming enlarged with the juice imbibed, she lays some 

 hundreds of eggs. The bud, meanwhile, instead of growing in 

 length, becomes fleshy, and this fleshiness is communicated to the 

 leaves. The consequence is an arrested bud, into the recesses of 

 which, the young issuing from the cluster of ova on the outside of 

 it, betake themselves, and soon become closed in by the increased 

 irritation occasioned by their presence in its interior." In every 

 swamp you may have seen this curious dwarfing of the spruce. 



In this class of diseases, produced by mechanical injuries, I 

 should place all wounds caused by bruises or by the pruning knife, 

 all loss of sap occasioned by pruning at the wrong time, and 

 numerous affections arising from violence in transplanting. But 

 most of these maladies have been well studied by you in a practical 

 manner, and therefore I will not occupy your time with their 

 consideration. 



The second group of plant-diseases comprises all those produced 

 by or accompanied by flowerless parasites. Let us first examine 

 the difference between a flowering and a flowerless plant, and then 

 notice some characteristics of the latter. The plants which you 

 cultivate on the farm, such as corn, turnips, and the various forage 

 grasses, are known to be reproduced by seeds through the agency 

 of the flower. These we term phenogamic or flowering plants. 

 There are others forming numerically a larger class, which are 

 reproduced not by seeds, but by spores formed without any 



