539 



mere examination of chemical conditions ( ferent from that of the majority of plants 



in this climate ; the particular irritability- 

 there denotes its constitution; it is the con- 

 stitution of the plant which causes it to 

 pass through these changes under differ- 

 ent conditions from other plants, scarcely 

 distinguishable probably in the majority of 

 their characters. There is nothing known, 

 and probably nothing to be known, in the 

 case of ChickweeJ, to distinguish it che- 

 mically from many other common weeds, 

 but it is the peculiarity of the constitution 

 of that species, that it passes through these 

 changes under different conditions. Abun- 

 dant illustrations of these peculiarities 

 might be readily furnished, with which it 

 is scarcely worth while to take up your 

 time; for instance, the late flowering of 

 the Ivy in November, the flowering of the 

 Hellebore or Christmas Rose in January, 

 and so on, as contrasted with the summer 



and changes, and take into account the 

 phenomena of life. The phenomena of 

 life as regards plants may be called the 

 phenomena of organization — that is, the 

 phenomena presented by the conversion 

 of mineral or dead matter into organs. — 

 Now, the difference between organs and 

 substances — those parts or constituents 

 which distinguish live things from mineral 

 or dead substances — lies in the circum- 

 stance, that while in substances we have 

 what we may call merely qualities, in the 

 organs we have what are called functions. 

 The qualities are, as it were, passive cha- 

 racteristics ; and functions are active cha- 

 racteristics — manifestations of constant, or 

 at all events, periodical activity, in the 

 presence or manifestations of which we 

 distinguish the Ibrce whi(*h we call the vi- 



tal force. This continued activity, more 

 or less independent of external causes 

 resulting in continued or periodical change, 

 is the sign of the existence of this vital 

 force — that force whose laws form the 

 subject of vegetable })hysiology. The 

 collective functions of a plant or animal 

 constitute its life. I will now confine my- 

 self to the consideration of the life of 

 plants, consisting in the performance of 

 their collective functions by the different 

 organs. Now, these functions collective- 

 ly constituting the life, exhibit what we 

 call the constitution. When we say that 

 a plant has a particular constitution, we 

 mean that it performs its functions in a 

 particular way. It is to the examination 

 of the constitution not only of plants, but 

 of the organs or parts of plants, to which 

 vegetable physiology has especially to di- 

 rect its attention; and I wish to-day briefly 

 to indicate some points of importance in 

 regard to this subject — the constitution of 

 plants or their organs. 



In the first place, I would direct atten- 

 tion to the phenomena illustrating the ex- 

 istence of constitutions in species or kinds 

 of plants. The phenomena which prove 

 these are familiar to every one. Every 

 one knows, for example, that Chickweed 

 flowers in the winter season, when the 

 great proportion of the plants growing in 

 this country are unable not only to form a 

 flower or seed, but even to })ut forth their 

 leaves. The irritability (to use a some- 

 what hypothetical expression) or the vital 

 activity of this plant is evidently vei-y dif- 



and autumn flowers, forming the great 

 mass of our native and cultivated vegeta- 

 tion. These peculiarities of constitution 

 are not entirely explicable by the view 

 which has been taken by some authors, that 

 plants are dependent for their growth upon 

 a certain supply of heat given to them. — 

 Some years ago M. Boussingault publish- 

 ed some interesting researches, in which 

 he showed that many plants, especially 

 cultivated plants, required a certain sum 

 of heat for their perfect development ; — 

 that between the time of the germination 

 of the seed and the ripening of the fruit, 

 or the production of the grain, the plant 

 required to receive a certain amount of 

 heat. He measured this heat by taking 

 the daily mean temperature and multiply- 

 ing it by the number of days, and he found 

 that the plants received pretty nearly the 

 same amount of heat in very different cli- 

 mates. Where the heat was greater the 

 plant was perfected in a smaller number 

 of days ; where the heat was less a great- 

 er number of days was required. For in- 

 stance, in Alsace, where his own property 

 was situated, he found that Barley would 

 be perfected in ninety-two days, where 

 there was an average temperature of 19 

 deg. of the French thermometer, giving a 

 total of 1,748 deg. He then obtained the 

 Date from Cumbal, situated under the 

 equator, where the plant was grown be- 

 tween June and November, instead of be- 

 tween May and August ; the daily tempe- 

 rature was there (it being in the colder 



