Another point which it is important to keep in view, is that instructions should be studied, rather than copied, in their application to practice. No instructions can be given that should be blindly and implicitly followed. The circumstances under which plants are placed are varying every day, and even every hour, and, to be successful, horticultural practice must be varied also. It must, however, be varied according to principle. But even what are regarded as established laws and principles should not be heedlessly followed; to be truly successful, a man must not only be a practical enthusiast and a keen theorist; he must also be a skilful experimentalist: his experiments and their results, if carefully watched, deduced, recorded, and studied, will serve to guide him for the future. APPENDIX. _On Heating, Ventilating or Aerating, and Covering._ Since the publication of the first edition of this work in 1844, the views expressed in the second chapter, with reference to structures best adapted for Cucumber culture in the winter season, have met with much corroborative support. Respecting the questions of heating, ventilation, and covering, a few more words may be added. I have before recommended hot water tanks for supplying bottom heat, with attached pipes for the circulation of hot water to warm the atmosphere. I can see no reason for recommending any other arrangement now; for the experience of successive years goes to show that hot water, applied on sound principles, is, above all other means of heating, effective in its operation; and as to the question of expense, raised as an objection to it by some, it is sufficient to say, that, although one hot water apparatus may be fitted up in an expensive manner, another may be rendered perfectly successful in its operation, at the same time that it is extremely simple in its arrangements, and correspondingly inexpensive in its cost.