100. One of the most essential of the alterations which occur in fruits during ripening is, the decomposition or dissipation of the water that they attract from the stem. A diminished supply of water will, under equal circumstances, produce an accelerated maturation, because less time will be required to decompose or dissipate this element; and, on the other hand, an excessive supply of water will retard or prevent ripening, in consequence of the longer time required for the same purpose.

101. Seeds are affected by all circumstances that affect the fruit, which, indeed, as has been already stated, appears to be created for their nutrition and preservation. In general, the fruit attracts organisable matter from the stem through the stalk, and the seed from the fruit through its placenta *; and this accounts, independently of

water, a sugar like that of Grape sugar is the result; that the gum of Peas, placed with oxalic acid in a temperature of 125°, is converted into sugar; that the gum obtained from starch, mixed with the juice of green Grapes, renders it saccharine; and, finally, that tartaric acid, assisted by heat, produces the same effect; which is what causes most fruits to become sweet when cooked. (De Candolle, Phys. Veg., p. 585.)

*The placenta is a soft part of the interior of a fruit, upon which the seed is formed. It is composed of thin-sided parenchyma, the most absorbent of all the forms of tissue, and is in communication, by its whole surface, with the parenchyma of the fruit.

other causes, for the importance of the fruit to the seed.

102. When the seed is ripe it is dry, all its free water being parted with; and its interior is occupied by starch or fixed oil, or some other such substance, together with earthy matters. It would seem that, so long as these secretions remain undecomposed, so long does the vitality of the seed continue unimpaired; and hence the great age at which certain kinds of seeds have been found to grow. But, as it is difficult to prevent their decomposition, so is it difficult to preserve seminal vitality for any considerable time; and the dif ferences found in the duration of the growing powers of seeds probably depend principally upon chemical differences in their constituent parts. Oily seeds, which readily decompose, are among the most perishable; starchy seeds, which are least subject to change, are the most tenacious of life.

103. Warmth, moisture, and an excess of oxygen, but especially warmth and moisture, while they are the greatest causes of germination, are pro

Not to speak of the doubtful instances of seeds taken from the Pyramids having germinated, Melons have been known to grow at the age of 40 years, Kidneybeans at 100, SensitivePlant at 60, Rye at 40; and there are now growing, in the garden of the Horticultural Society, Raspberry plants raised from seeds 1600 or 1700 years old. (See Introduction to Botany, ed. 3. p. 358.)

bably, on that same account, the chief causes of death. Seeds remain dormant so long as the proportion of carbon peculiar to them is undiminished; water is decomposed by their vital force (14.); and its oxygen, combining with the carbon, forms carbonic acid, which is given off. The effect of access of water is, therefore, to rob seeds of their carbon; and the effect of destroying their carbon is to deprive them of the principal means which they possess of preserving their vitality.

104. Although a seed, if fully formed, is in all cases capable of perpetuating its race, yet there is a difference in the degree to which this capability extends. All seeds will not equally produce vigorous seedlings: but the healthiness of the new plant will correspond with that of the seed from which it sprang. For this reason, For this reason, it is not sufficient to sow a seed to obtain a given plant: but, in all cases where any importance is attached to the result, the plumpest and heaviest seeds should be selected, if the greatest vigour is required in the seedling; and feeble or less perfectly formed seeds, when it is desirable to check natural luxuriance. It is apparently for this reason, that old Melon seed is preferred to new; for the latter would give birth to plants too luxuriant for the small space in which the Melon can be cultivated, under the artificial circumstances required in this country.

105. As both fruit and seeds are maintained at the expense of the leaves, the destruction of the former, when young, will enable the latter to store up against a succeeding season, for the support of future flowers, all that organisable matter which the fruits and seeds destroyed would have otherwise consumed.

CHAP. VII.

OF TEMPERATURE.

Limits of Temperature endurable by Plants. Effects of a too high Temperature - Of a too low Temperature. — Frost.-Alternations of Temperature.-Day and Night.— Winter and Summer.- Temperature of Earth and Atmosphere.

106. THE extreme limits of temperature which vegetables are capable of bearing, without destruction of their vitality, have not been determined with precision; it is, however, known, that, on the one hand, certain seeds may be boiled without being killed, and that, on the other, they are capable of bearing many degrees of freezing without suffering. In like manner, some plants are found to endure the most intense cold known upon the globe, while others sustain, occasionally, a tem

G

perature as high as 140°, as was observed by Dr. Coulter on the banks of the Rio Colorado.* The number of plants, however, capable of sustaining such extremes of temperature, is small, and the greater part of the species known to us are proved to exist within the limits of 32° and 90°. What amount of temperature a given species will prefer, under different circumstances, seems reducible to no general rule, but has to be determined experimentally in each case, or is judged of by the known climate of which a plant may be a native. It is probable that every species has a constitution better suited to some particular amount of temperature than to any other, although it can bear a greater or less degree without sustaining injury.

107. Although many plants will live in a temperature much below that of freezing, yet no plant is able to grow unless the temperature is above 32°, for physical reasons that require no explanation. When temperature rises, the air contained in the minute cells of plants expands, the fluids become thinner, the excitability of the tissue is aroused, and, at the same time, insensible perspiration is commenced, the effect of which is to bring into play the absorbing powers of the roots, and thus to set the machinery of vegetation in action. The

• The temperature borne by Oscillatorias in thermal springs is much higher than this; but no such power is possessed by cultivable plants.