VEGETABLE PHYSIOLOGY. 



therefore, the attention of vegetable histologists 

 has of late years been specially directed, and the 

 results have appeared in the form of certain theories 

 which have given rise to much angry discussion. 

 There appear to be four modes in which vegetable 

 cells are multiplied namely, by nuclei, by division, 

 by gemmation, and by conjugation. 



GENERAL INTEGUMENT AND ITS APPENDAGES. 



Submerged plants have the cells of their leaf- 

 tissue directly exposed to the action of the sur- 

 rounding water ; but land-plants usually have all 

 their organs invested in an epidermis or skin, 

 which regulates transpiration, and prevents their 

 tissues becoming dried up. The surface of this 

 epidermis, again, is covered by a very thin struc- 

 tureless layer, called the cuticle. The epidermis 

 is composed of cellular tissue; but the cells are 

 pressed closely together, and flattened, and they 

 are often filled with air instead of water. The use 

 of the epidermis is to retain a sufficiency of mois- 

 ture in plants; for should the delicate membrane 

 of which the cells of their tissue are composed 

 become so dry as to lose its elasticity, the different 

 organs would be unable to perform their proper 

 functions. On this account, its thickness is curi- 

 ously adapted to the conditions under which a 

 plant grows. Plants of very hot countries are 

 supplied with three or even four layers of dense 

 external tissue, in order that the moisture may be 

 retained, notwithstanding the excessive heat and 

 dryness of the climate. But it is necessary that 

 the tissues of plants should not be shut out from the 

 free action of the atmosphere, whence a large pro- 

 portion of their food is derived. They are therefore 

 provided with peculiar breathing-pores, or stomata, 

 by which air and fluid enter, and from which fluid 

 transpires. In mosses, where the leaves usually 

 consist of a single layer of cells, stomata are con- 

 fined to the fleshy base of the fruit ; but in flower- 

 ing-plants they chiefly occur on the under surface 

 of the leaf; and this is especially the case in many 

 evergreen shrubs, which are thereby enabled to 

 benefit by the moist exhalations from the soil and 

 herbage beneath, without suffering from a scorching 

 sun. This law is reversed where the habits of the 

 plant require such an adaptation. Water-lilies and 

 other plants whose leaves float on the surface of 

 the water or lie flat on the soil, have no stomata 

 on their under surface, but are supplied with an 

 increased number on their upper surface, which 

 alone is exposed to the action of the atmosphere. 

 The following calculation of the number of stomata 

 in the leaf of the royal water-lily, will serve to 

 indicate the extremely minute size of these bodies 

 and the great numbers of them required by plants : 

 each stomate measures the ^loth part of an inch 

 in diameter; one square inch of surface contains 

 139,843 stomata; so that one ordinary sized leaf 

 of this plant, with a surface of 1 850x18 square 

 inches, contains upwards of twenty-five millions 

 of stomata (25,720,937). 



Hairs are minute prolongations from the epi- 

 dermis, and are found upon almost every part of 

 plants. Sometimes they cover the whole of the 

 leaf, and at others they are only found on one 

 surface. They are described in general terms as 

 downy, silky, hirsute, bristly, ciliate, &c. according 

 to their aspect and mode of arrangement The 

 hairs of plants are exceedingly variable in size 



and form, and are either unicellular or multicellular 

 consisting of one or of many cells. In either 

 case, they may be simple or branched. There are 

 branched unicellular hairs in Cruciferae, and beau- 

 tiful stellate hairs in many plants. The sting of 

 the nettle is a modification of the hair, being a 

 conical tube with a basal bulb, filled with irritant 

 fluid, which shews singular movements under the 

 microscope. The tube is sharp-pointed, and sur- 

 mounted by a little curved knob ; when the plant 

 comes in contact with the hand, this knob is 

 knocked off; the sharp point which it protected 

 now penetrates the skin; and the pressure upon 

 the conical sting causes the irritant fluid which it 

 contains to be poured out into the wound. The 

 whole phenomenon is strikingly similar to the 

 mode of action of the animal sting. 



Glands. Although some have stated that there 

 is no true process of secretion in plants analogous to 

 that of animals, still the latest researches seem to in- 

 dicate that such a process does exist, and that it is 

 performed by special organs usually modifications 

 of the epidermis itself or of its appendages. Thus, 

 in the Cinchonas of South America, we have con- 

 ical glands, which pour out a gum-resinous matter 

 on their free surface. The same kind of glands 

 are found in the bedstraws of our hedgerows. The 

 honey of many flowers also appears to be a true 

 secretion, poured out upon a free surface of the 

 tissue by special cells, which are not analogous to 

 the fat-cells of animals, to which the so-called 

 vegetable secretions have been likened. 



Besides the above-mentioned organs, there are 

 prickles, thorns, and spines. Prickles may be 

 called hardened hairs, as they are merely indurated 

 expansions of the epidermis, without any woody 

 fibre ; and they may be detached from the branch 

 which bears them without laceration. Examples 

 of prickles are well seen in the rose and bramble. 

 Thorns differ from prickles in being formed partly 

 of woody fibre ; and they cannot be detached from 

 the branch which bears them without lacerating 

 its vessels. They have their origin in buds, and 

 are the result of an arrestment of development, 

 being formed instead of leaves and branches. 

 Examples in hawthorn and sloe. Spines resemble 

 thorns in every respect, except in being found on 

 the leaves and stems of herbaceous plants ; while 

 thorns only grow on the trunk and branches of 

 woody plants. When spines grow on leaves, they 

 are always found on the veins which are extensions 

 of the woody fibre. Example, holly. 



ORGANS OF NUTRITION. 



The organs of nutrition are the root, the stem 

 and its branches, and the leaves; and of these 

 organs, the root and the leaves, or some modifica- 

 tion of them, exist in every flowering-plant, as the 

 vital functions cannot be carried on without them. 



The root (radix in Latin) is commonly defined 

 to be that part of a plant wnich attaches itself to 

 the soil where it grows, or to the substance on 

 which it feeds, and is the principal organ of nu- 

 trition. Exceptions to this definition occur, as in 

 the case of some plants which grow floating loosely 

 in water, as duck-weed, as well as in the case of 

 others having no root at all. As the nourishment 

 of a plant is derived from the earth, the root is 

 that part which grows in an opposite direction to 

 the stem, and is buried in the ground. It is the 



