EXCITABILITY NECESSAKY TO SUCCESS. 295 



results attended other trials. It is probable that the failures 

 in the process were ascribable to what seems to be an imper- 

 fection in his process. If, instead of merely coTering the cut 

 end of the cutting with collodion, he had in some cases covered 

 the cutting itself, as far as it was plunged in the soil, he would 

 perhaps have had stiU greater success. There are two kinds of 

 cuttings which are likely to demand this treatment — the one of 

 succulent or very soft plants, which absorb abundantly through 

 their skin; and the other of hard-wooded plants, of little 

 substance, which become exhausted by the drying-up of their 

 organizable matter before roots can be formed. 



It is known that plants possess some quality analogous to 

 animal irritability, to which, for want of a better, the name of 

 excitability has been given. In proportion to the amount of 

 excitability in a given plant is the power which its cuttings 

 possess of striking. The great promoter of vegetable excita- 

 bility is heat. Therefore the more heat a given plant has been 

 exposed to, within certain limits, the more readily its cuttings 

 wiU strike root. This explains what seems to have puzzled 

 Mr, Neumann. " The young wood," he says, " of trees grow- 

 ing iu the open air will not do for cuttings ; and yet, if those 

 same trees are forced in a hothouse, their cuttings are almost 

 sure to succeed. Physiologists have not discovered the cause 

 of this singular phenomenon. Many attempts have been made 

 to strike cuttings from trees growing in the open air, but with- 

 out success. If, for instance, we take an old branch of 

 Pawlovnia imperialis, in the inonth of March, and put it into 

 earth, or tan, or even water, in a stove with a good tempera- 

 ture, its buds wiU soon appear, and as soon as they are long 

 enough to make herbaceous cuttiags, will strike freely. But if 

 you take them as they form upon a tree exposed to the open 

 air, they wiU not grow at all." Vegetable excitability, a most 

 important attribute, but little thought of, is the obvious expla- 

 nation of this fact. 



In addition it seems desirable to add some observations upon 

 the object of additional precautions often taken by gardeners. 



Cuttings are covered by bell-glasses, whose edge is pressed 

 into the earth. This is for the purpose of preserving a 



