140 



SCIENCE GOSSIP. 



THE STRUCTURE OF PLANTS. 



A CHAPTER IN ORGANIC EVOLUTION. 



By Rudolf Beer. 



IN the present note I propose to deal rather 

 with the principles which underlie anatomical 

 facts than to enumerate these facts themselves. 

 I have attempted to give a sketch of the prob- 

 able biological conditions that existed in the 

 past, and to trace to some extent the influence 

 which these have had upon the internal structure 

 of plants. My aim has been to show that the 

 science of their anatomy is not the bare record of 

 isolated facts, but that it employs these facts as 

 tools wherewith to read the past history of the 

 vegetable kingdom, or to attain a closer under- 

 standing of the vital activities of plant-mechanism. 

 Let me take our imaginations back through the 

 millions of years that separate us from the time 

 when the surface of this great earth was one dead 

 waste except the inland waters. It was most 

 probably in the water which collected in the 

 hollows of the land that the primitive organisms 

 arose. We have no record to show what these first 

 living creatures were actually like, but there are 

 organisms still existing that we have reason to 

 believe are their lineal descendants, and through 

 the study of which we can make fairly probable 

 guesses at the changes that have taken place. 



There is a little alga named Chlamydomonas, 

 occurring very frequently in standing water, that 

 will serve as the type of a fairly primitive form. 

 It is, I need hardly say, a microscopic plant in- 

 visible to the naked eye. Its body is somewhat 

 egg-shaped in outline, and consists of that half- 

 fluid substance which is familiar under the name 

 of protoplasm. Within this little, egg-shaped 

 mass of living matter lies a central, spherical body, 

 the nucleus. Around the whole organism a firm 

 envelope or cell-wall is wrapped, which protects 

 the delicate living mechanism within from the 

 rough treatment it is likely to receive during 

 its existence. The whole structure — protoplasm, 

 nucleus, and cell-wall together — constitutes a single 

 cell. 



Before we leave this creature we should observe 

 two further facts. One is that a very great deal of 

 its protoplasm is coloured green by a substance 

 known to the botanist as chlorophyll. ' This green 

 pigment has the remarkable faculty of catching 

 and holding the rays of sunlight which fall upon 

 it, and, by virtue of this property, of endowing the 

 protoplasm it colours with the power of manu- 

 facturing food materials for the plant from simple 

 gases and water. The other fact that should be 

 noticed is that Chlamydomonas is a motile form, 

 rapidly hurrying across the field of the microscope. 



It requires careful observation to show how this - - 

 movement is effected. Under favourable condi- 

 tions it will be seen that two delicate protrusions- 

 of the protoplasm penetrate two tiny perforations 

 at the narrow end of the cell wall, and that these 

 protoplasmic "cilia," as they are called, lash the 

 water continually and propel the organism forwards- 

 as though by the beat of a pair of oars. 



A higher state of organisation is illustrated by 

 the very common water-weed Spirogyra. The 

 cells of this creature are not egg-shaped, but 

 cylindrical ; they are not motile, but fixed ; further,, 

 they do not carry on an isolated existence, each 

 for itself, but remain joined together by their ends- 

 so as to form long threads. The portion of the- 

 protoplasm which is coloured green is more sharply 

 limited from the colourless portion, and it is shapecl 

 like a winding spiral running from end to end of 

 each cell. Another organism not uncommon in 

 the ponds of bogs and heathlands is Pediastrum. 

 Here the cells are fixed together, not only by their 

 ends, but by their sides, so as to form a little plate 

 of cells. In Diatyospliaeria, an inhabitant of 

 tropical seas, we have a form in which the cells 

 adhere together on all sides, making a mass of 

 cells. 



The organisms we have considered up to the 

 present will serve to exemplify the general nature- 

 of the first plants which existed on this globe. 

 They were, as I have already pointed out, ex- 

 clusively fitted for an aquatic existence. The 

 reader's own observations will no doubt have 

 shown, over and over again, that a pond or lake 

 left to itself will gradually become more and more 

 crowded with water-weeds such as those above 

 mentioned. In the far-away geological times the 

 still waters of the ponds, untroubled by man or 

 ducks, would soon overflow with life. In such an 

 overstocked pond the struggle for existence between 

 the innumerable organisms would be exceedingly 

 keen. Light, air, and inorganic food materials 

 would be at a premium. Any slight advantage 

 which an individual might possess in its structure,, 

 enabling it to procure the necessaries of life more 

 readily than its fellows, would weigh heavily in 

 its favour in the fierce battle for existence. It 

 would grow more vigorously, and produce more 

 and stronger offspring than any of the other forms. 

 Step by step this variety would take precedence 

 over all its fellows, and it would slowly crowd 

 them out of the limited area of the pond. More- 

 over, its spores would be lifted by the wind or 

 borne on streamlets which flow from the pond, and 



