DISEASES OF PLANTS. 



397 



Thus, of the plants examined, those having white flowers presented 

 the larger proportion of odoriferous species. The orange and brown 

 coloured flowers often gave a disagreeable odour. In examining 

 numerous species from various natural orders, they found that out of 

 100 species of 



Nymphseaoeffi 

 Eosacese . 

 Primulacese 



ginacefe 

 ConvolvulaoesB 

 Eanvmoulacese 

 Papaverace^. 

 Campamilacese 



22 were odoriferous. 



13 



12 



6 



4 



4 



2 



1. 



6. — Diseases of Plants. 



Great obscurity attends this department of botany, and much 

 remains to be done ere a system of vegetable nosology (votros, disease) 

 can be completed. It is, however, of great importance, whether we 

 regard its bearing on the productions of the garden or the field. Some 

 have divided the diseases of plants into general, or those affecting the 

 whole plant, and local, or those afiecting a part only. A better 

 arrangement seems to be founded on their apparent causes, and in this 

 way they have been divided by Lankester into four groups. 1. 

 Diseases produced by changes in the external conditions of life ; as 

 by redundancy or deficiency of the ingredients of the soil, of light, 

 heat, air, and moisture. 2. Diseases produced by poisonous agents, 

 as by injurious gases, or miasmata in the atmosphere, or poisonous 

 matter in the soil. 3. Diseases arising from the growth of parasitic 

 plants, as Fungi, Dodder, etc. 4. Diseases arising from mechanical 

 injuries, as wounds and attacks of insects. 



Plants are often rendered liable to the attacks of disease by the 

 state of their growth. Thus, cultivated plants, especially such as be- 

 come succulent by the increase of cellular tissue, appear to be more 

 predisposed to certain diseases than others. Concerning the first two 

 causes of disease very little is known. Absence of light causes blaneh- 

 ing, which may be looked upon as a diseased state of the tissues. 

 Excess of light may cause disease in plants whose natural habitat is 

 shady places. Excess of heat is sometimes the occasion of a barren or 

 diseased state of some of the organs of the flowers, and frost acts pre- 

 judicially on the leaves, stem, and floVers. By excess of moisture a 

 dropsical state of the tissue is induced. A curious instance of 

 mechanical injury acting on plants is given by Mr. Berkeley. He 

 states that the injury to the tops of the branches of Araucaria 

 imbricata is caused by the shoots coming in contact with their neigh- 



