PLANKTON 



is a pointed support E, and on the 

 other a pencil or tracing point, F. 

 The latter arm has on it a graduated 

 cylinder, D, which rolls round as the 

 tracing pointer is moved along the 

 perimeter of the area to be mea- 

 sured. A vernier, H, and a hori- 

 zontal disk, G, worked in conjunc- 

 tion with D, enable the area of 

 the figure to be measured or calcu- 

 lated See Vernier. 



Plankton (Gr. plankto*, wan- 

 dering ). Term used for the drifting 

 or swimming organisms of lakes, 

 rivers, and seas. The plankton of 

 the latter is the basis of oceanic 

 life, and the term was first used by 

 Victor Hensen to indicate those 

 organisms which were at the mercy 

 of every current, their swimming 

 powers being too feeble to make 

 any headway agiinst a flow of 

 water. The colour of certain seas 

 is entirely due to these minute 

 organisms, which include plants as 

 well as animals, and exist in count- 

 less myriads. The study of the 

 distribution of plankton is impor- 

 tant, for the organisms are the 

 basic food supply of many fish, 

 e,g. herrings, whose migratory 

 movements can be consequently 

 better understood. Plant plankton 

 includes diatoms, algae, etc., and 

 animal plankton the jelly fishes, 

 foraminifera, radiolaria, siphono- 

 phora, certain Crustacea, etc. See 

 Nekton. 



Planquette, ROBERT (1848- 

 1903). French composer. Born in 

 Paris, July 31, 1848, he studied at 

 the Conserva- 

 toire there. He 

 composed a 

 large number 

 of o p e r ettas, 

 including Les 

 Cloches de 

 Corneville and 

 Rip van Win- 

 k 1 e, which 

 Robert Planquette, gained much 

 French composer popularity in 

 both France and Britain. Other 

 operettas included Nell Gwynn and 

 The Old Guard. He died in Paris, 

 Jan. 28, 1903. 



Plant (Lat. planta, sucker or 

 shoot). Word usually indicating 

 in its everyday popular use a herb. 

 In the broader botanical sense, 

 however, it covers all vegetable 

 organisms, not only trees, shrubs 

 and herbs, but the ferns, mosses, 

 liverworts, seaweeds, fungi, and 

 even the minute single-celled 

 organisms that are so like the sim- 

 plest forms of animal life. Just as 

 the higher animals have well- 

 recognized divisions of the body, 

 as trunk, head, and limbs ; so the 

 higher plants have equally distinct 

 parts, as root, stem, leaves, flower, 

 and fruit. All these parts are sub- 



6 1 86 



ject to considerable modification, 

 and upon these they are classified 

 into orders, genera, and species. 



The root as a rule burrows into 

 the earth and bears no leaves, its 

 functions being to attach the plant, 

 and to provide it with water and 

 mineral salts for food. The stem 

 takes a more or less upward direc- 

 tion above the soil and bears 

 leaves. It may be composed en- 

 tirely of soft cells and easily 

 crushed by a little pressure ; or 

 of hard cells forming a more or 

 less massive enduring trunk. 

 Such stems, though they start 

 like the soft green ones of the 

 herbs, develop a jacket of specially 

 modified cells, which gradually 

 becomes thick bark. 



The leaf is normally a thin, flat, 

 green expansion of soft cells which 

 contain the green chlorophyll essen- 

 tial to the nutrition of the plant. A 



Planimeter. Diagram showing Amsler's 

 planimeter. See text 



strand of vascular tissue runs from 

 the stem down the centre of the 

 leaf (midrib) and branches to the 

 edges. The leaf-surface is studded 

 with hundreds of breathing pores 

 (stomata) through which is ab- 

 sorbed the carbonic acid gas of the 

 atmosphere. The chlorophyll is 

 able to separate the carbon con- 

 tained in the carbonic acid gas, 

 and to unite it with the oxygen 

 and hydrogen contained in the 

 water from the roots to form starch 

 grains. These are afterwards con- 

 verted into sugar. Leaves are more 

 subject to modification than any 

 part of the plant, and may become 

 needle-like spines as in cactus and 

 furze, or tendrils as in many climb- 

 ing plants. They are modified into 

 sepals and petals to protect the 

 reproductive organs and form the 

 flower. The stamens and carpels 

 (pistils) which contain the sexual 

 elements the pollen and ovules 

 are also greatly modified leaves. 



Plant Intelligence 



Though plants lack the brain and 

 nervous system possessed by the 

 higher orders of animals, they have 

 undoubted powers of communicat- 

 ing impressions from one part of 

 the plant body to another by means 

 of protoplasmic threads connecting 

 cell with cell, and of responding to 

 these impressions. They are sensi- 

 tive to light and touch ; the tips 

 of the roots have the faculty of 

 food-finding, for Darwin compared 



PLANT 



the root-tip to a brain ; and they 

 have a good deal of power of 

 adaptation to their environment. 

 Many plants seem to have inherited 

 memory of seasonal changes that 

 will affect them, and to prepare in 

 advance to meet altered conditions. 



Plants that depend upon insects 

 for the fertilisation of their ovules 

 act in a way that would be classed 

 as intelligent if performed by 

 animals. Some, when fertilisation 

 has been effected, notify the fact 

 to the insects concerned by a 

 change in the colour of the corolla, 

 or by its wilting, or even by shed- 

 ding it bodily. In some flowers, 

 when the pollinating insect arrives, 

 the stamens spring up suddenly 

 and dust it with pollen in the exact 

 spot that will come in contact with 

 the receptive stigma of- the next 

 flower visited. Bulbs, which begin 

 life as seedlings on the surface of 

 the ground, afterwards dig them- 

 selves in to a depth where they 

 will be safe from frost and drought. 

 The frogbit, which floats on the 

 surface of ditches, appears to have 

 inherited knowledge of what would 

 happen to it in winter when the 

 water is frozen ; so in autumn it 

 forms several buds or bulbils into 

 which it packs all its useful sub- 

 stance, and these drop into the 

 mud at the bottom, from which 

 they arise in spring and put forth 

 leaves and flowers at the surface of 

 the water. 



Geographical Distribution 



Theoretically, every kind of 

 plant must have originated in 

 one particular region, from which 

 in course of time it must have 

 spread gradually to those countries 

 in which also it is found to-day. 

 Distribution follows certain lines 

 because of the continuous suit- 

 ability of climate and soil. Con- 

 tinuity is interrupted because the 

 suitable soil is already occupied by 

 a more vigorous species. Seas or 

 mountain-chains may be im- 

 passable for one species. Various 

 agencies assist in the spread of 

 plants from their original homes, 

 among them being winds, rivers, 

 ocean-currents, birds, and beasts, 

 mostly as seed -carriers. 



Sometimes plants have passed 

 from continent to continent by 

 formerly existing land-bridges. 

 Many plants found in Cornwall and 

 Ireland also occur only in W. 

 France and the Iberian Peninsula. 

 It has been concluded that these 

 plants reached Ireland and Corn- 

 wall by land, at a period when 

 Ireland was joined to England, and 

 England to France and Portugal. 

 See Botany ; Bud ; Crescograph ; 

 Flower ; Leaf ; Pistil ; Plant, colour 

 plate, facing page 6151 ; Stamen, 

 etc. ; consult also Plant-geography 



