No. 8. 



Mildew. 



251 



growth and expansion of the plant, and enable 

 it to perform the functions designed in its 

 creation ; and the great object would seem to 

 be, to cause it to perfect its seed and con- 

 tinue its species. When this deposit of nu- 

 triment is made, the water, which was its 

 vehicle of conveyance, is thrown out of the 

 plant as excrementitious by proper exhaling 

 vessels, and is dispersed, in the form of vapour, 

 in the atmosphere, and the vessels of circu- 

 lation, which are in continual action, intro- 

 duce continued supplies of similar nutriment 

 duly prepared for deposit, and throw off the 

 water as before, so that there is never a va- 

 cuum in the plant. 



This process is in continual action till the 

 plant is perfected, and the quantity of water 

 thrown off during the progress of vegetation 

 is almost incredible, as would appear by some 

 very accurate experiments made to indicate 

 the amount. Of 15 parts of water taken 

 up by some plants, 13 are transpired, and of 

 the lowest on the scale, of 5 parts taken up 

 4 are discharged by exhaling vessels. 



Wheat and other plants, when they have 

 acquired their full growth, commence the in- 

 teresting business of perfecting their seeds, 

 in order to perpetuate their kind ; and then 

 nature brings all her powers into requisition 

 to effect this remarkable process. The ves- 

 sels of the plant are distended with the pro- 

 per fluid making its way to the seed-vessel, 

 which has been duly prepared to receive it, 

 which is there deposited in the form of a 

 milky juice, when the water, which has been 

 the vehicle for its conveyance, is discharged 

 through exhaling vessels into the atmosphere, 

 and another supply from the same source is 

 constantly in the rear, to be disposed of in a 

 similar manner ; and so the process goes on 

 — provided there is no unfortunate interrup- 

 tion from external causes — till each grain is 

 filled with farina, when, the great work being 

 completed, the circulation ceases to be car- 

 ried on, and the whole is dried and hardened 

 for preservation. 



When this process is going forward, it will 

 be perceived that a vast proportional quantity 

 of water must be constantly discharged into 

 the atmosphere, otherwise, space would not 

 exist in the hull of the grain for additional 

 supplies of the diluted nutriment, which is 

 continually arriving at its destined deposito- 

 ry ; but should the atmosphere at this critical 

 period unfortunately be saturated or sur- 

 charged with moisture, as has been before 

 hinted, it will be unable to take up and carry 

 away that which the grain must necessarily 

 ■part with, and which is now an incumbrance 

 to it, in order to make room for an additional 

 supply of the fluid which would increase the 

 deposit of farina. This inability of the air 

 to take up an additional load of moisture, 



under the circumstances of its previous satu- 

 ration, prevents it also from carrying off the 

 heat from the wheat, so that the temperature 

 of the whole plant is increased much above 

 the proper standard of its healthy action; for 

 the temperature of plants that transpire 

 moisture freely is constantly kept many de- 

 grees cooler than the surrounding atmosphere, 

 or bodies destitute of vital action. Tihis re- 

 tention of excrementitious moisture suspends 

 the circulation, for it can't move unless it can 

 get vent, and that and the expansion occa- 

 sioned by increase of temperature produce 

 congestion, and burst the vessels of circula- 

 tion, and discharge their contents into the 

 cellular tissue, destroy the vitality of the 

 plant, and leave the hull only partially filled 

 with farina. Heat, air and moisture, the 

 agents of putrefaction, being present, decom- 

 position begins, and the surliice of the plant 

 soon displays signs of decay. Tiiis destruc- 

 tive process first shows itself in the smallest 

 part of the stalk, near the head, which is of 

 most recent formation, and consequently most 

 succulent and tender, and most liable to rup- 

 ture. The rust is probably occasioned by the 

 heat and internal pressure enlarging the 

 pores of exhalation and discharging the sap 

 on to the surface of the stalk, and when eva- 

 poration carries off the moisture, the resi- 

 duum displays itself somewhat like the rust 

 of iron. After the rupture and discharge of 

 the sap-vessels, the surface of the plant is 

 covered with mucus which is adhesive, and 

 this will account for the seeds of fungi, which 

 are supposed to be floating in abundance in 

 the atmosphere, taking root and vegetating 

 in the decaying structure: and hence the 

 supposition, that fungi are the cause of mil- 

 dew. 



The presence of animalcula may be ac- 

 counted for on the same principle, for nature 

 is ever economical, and wherever animal or 

 vegetable substances are in the progress of 

 decay, mouths are found ever ready to con- 

 vert dead matter into food for living things, 

 so as to perpetuate the largest possible 

 amount of animated existence. On the 

 death of the plant, the tender succulent fibres 

 of the roots immediately decay, and on draw- 

 ing them from the ground, the appearance 

 of them has led many to suppose that they 

 had thus discovered the true cause of the 

 disease of the plant, when, in fact, it was 

 only the effect of its previous dissolution. 



It may be objected, that if the mildew is 

 the result of a general saturation of the at- 

 mosphere with moisture at a particular pe- 

 riod, that all wheat should be equally injured 

 by it ; when the fact is well known, that of 

 contiguous fields one will be destroyed and 

 the other remain uninjured. This apparent 

 contradiction is accounted for by the unin, 



