VITALITY OF SEEDS.

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or dissipate this constituent; 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.

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 organizable matter from the stem through the stalk, and the seed from the fruit through its placenta ;* and this accounts, independently of other causes, for the importance of the fruit to the seed.

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 differences found in the duration of the growing powers of seeds probably depend principally upon differences in their chemical constitution. Oily seeds, which readily decompose, are among the most perishable; starchy seeds, which are least subject to change, are the most tenacious of life.

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 forty years, Kidneybeans at a hundred, Sensitive-Plant at sixty, Rye at forty; and there are now living, in the garden of the Horticultural Society, Raspberry plants raised from seeds sixteenhundred or seventeen-hundred years old. The seeds of Charlock buried in former ages spring up in railway cuttings; where ancient forests are destroyed, plants appear which had never been seen before, but whose seeds have been buried in the ground; when some land was recovered from the Baltic sea, a Carex was found upon it, now unknown in that part of Europe. M. Fries, of Upsala, succeeded in growing a

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.

species of Hieracium from seeds which had been in his herbarium upwards of fifty years. Desmoulins has recorded an instance of the opening of ancient tombs, in which seeds were found, and on being planted they produced species of Scabiosa and Heliotropium. And many more such cases are on record, establishing conclusively that, under favourable conditions, the vitality of seeds is preserved for indefinite periods,

Warmth, moisture, and an excess of oxygen, but especially warmth and moisture, while they are the greatest causes of germination, are probably, on that same account, the chief causes of death. It seems as if seeds remain dormant so long as the proportion of carbon peculiar to them is undiminished; water is decomposed by their vital force; and it is believed that 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.

Be this as it may, it is incontestable that as soon as seeds begin to germinate, their vitality is exhausted and they perish, unless the seed is in a condition to continue its growth by obtaining sufficient food from surrounding media.

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, 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.

REMOVAL OF FRUIT BENEFICIAL.

105

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

LIMITS OF TEMPERATURE

CHAPTER VIII.

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.

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 temperature 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.

Although many plants will live in a temperature much below

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

EFFECT OF TEMPERATURE.

107

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 augment the absorbing powers of the roots, and thus to set the machinery of vegetation in action. The degree of temperature required to produce this effect is extremely variable in different species of even the same climate, and is, of course, much more variable between plants of different climates. For example, the common weeds called Chickweed, Groundsell, and Poa annua, evidently grow readily at a temperature very near that of 32°; while the nettles, mallows, and other weeds around them, remain torpid. In like manner, while our native trees are suited to bear the low temperature of an English summer, and, in most cases, suffer if they are removed into a country much warmer, such plants as the Mango, the Coffee, &c., inhabitants of tropical countries, soon perish, even in our warmest weather, if exposed to the open air.

When, in the case of a given plant, the temperature is permanently maintained at a much higher degree than the species requires, it is over-excited. If the atmosphere is preserved in a proportional state of humidity, the tissue grows faster than the vital forces of the plant are capable of solidifying it, its excitability is gradually expended, the whole of its organization becomes enfeebled, the vital functions. deranged, and a state of general debility is brought on.

are

According to Mr. Knight, the effect of an excessively high temperature is to cause, in unisexual plants, the production of male flowers only, while a very low temperature produces the contrary result. A Water Melon plant was grown in a house, the heat of which was sometimes raised to 110° during the middle of warm and bright days, and which generally varied, in such days, from 90° to 105°, declining during the evening to about 80°, and to 70° in the night; the air was kept damp by copious sprinkling with water, of nearly the temperature of the external air, and little ventilation was allowed. The plant, under these circumstances, grew with great health and luxuriance, and afforded a most abundant blossom; but all its flowers were male.