March, 1929 



EVOLUTION 



Page Seven 



Plant Evolution 



Bv FLORENCE DOWDEN WOOD 



PLANTS have undergone as complete evolution as have 

 the animals. There are three main lines of proof : 

 1) comparative study of existing plant forms; 2) specializa- 

 tion in reproduction; 3) fossil evidence. The lowest plants 

 started out from a common stem with the single-celled animal. 

 This single-celled organism, neither definitely plant nor animal, 

 could not only digest food in the animal fashion, hut could 

 also manufacture its ovi'n food in plant fashion. 



Any organism which can make food from the elements sur- 

 rounding it, would seem ideal at first glance. Plants from the 

 first, must have had this ability, but they have sacrificed the 

 power of locomotion, as well as the possibilty for a central 

 nervous system to guide them to adequate sources of food. 



In plants, as in animals, the tendency has been to grow 

 larger and larger, as much so as possible in each stage of 

 evolution, given the then existing degree of specialization 

 for food manufacture and reproduction. When size alone is 

 considered, plants have been able to attain that without much 

 in the way of specialization, as is seen in the enormous sea 

 weeds, some attaining a length of 600 or more feet. .\n ad- 

 ditional barrier, before attaining the goal of the modern seed 

 plant, is the lack of a satisfactory means of reproduction, as 

 all primitive plants require water in which the sex products 

 can swim to each other. In no case is there a mechanical 

 means provided for getting the sex cells together, except 

 as the cells themselves have the power of motion. The higher 

 plants depend on wind and insects for pollination. 



/. Euglena. the plant-animal. J. Pandorina, free swimming 

 colony. 3. Volvox, a large colony. 



.Assuming that we start with an undifferentiated "plant- 

 animal" organism as the plant's great-grandmother, we must 

 bring it up through the stages marked by the various levels 

 of existing plants till we reach the highest type of seed 

 plant. The first stage is Euglena (1). It is so small that a half 

 dozen individuals can be put in the space of an "i" dot. The 

 free-swimmmg plant-animal reproduces itself by simple divi- 

 sion. In its eagerness to attain higher things, the new- 

 organism does not have time to separate completely from 

 the old, resulting in a colonial form, the Pandorina (2) stage 

 now barely visible to the naked eye. This colonial form has a 

 tendency to grow larger. With further increase in size, 

 there is too little economy of energy, and a new advance 

 appears, a division of labor. Some cells are set aside for 

 reproduction, while the rest manufacture food, and propel 

 the organism about, as in the Volvo.v stage (3). The re- 

 productive products, "eggs" and "sperm" in this stage 

 are not essentially different in appearance from the original 

 one-celled plant-animal. 



Very soon this colonial form finds itself growing larger. 

 It must then do one of two things, either remain small 

 and free-swimming to avoid being broken apart by moving 

 water, or to find a quiet place to sit down and grow. Hav- 

 ing elected to lead a quiet life, the plant develops a set of 

 "holdfasts," which resemble roots but do not function as 

 such. These enable it to keep a firm grip on the rock or 

 mud bottom of the water, as in the kelp or sea weed stage (4). 



If the plant is to live on land, it must develop a system 

 to get water, nitrates, inorganic salts, and CO; frimi its 



environment, and be able to collect them together for 

 chemical rearrangement into foods. Also, it must develop 

 a system of reproduction which does not require water to 

 bring together its se.x products to produce a new organism. 

 A big advance is made in the moss stage of our evolving 

 plant. It has overcome some of the difficulties of food ab- 

 sorption. It has new absorptive root hairs on what we may 

 call the old "holdfast"; food elements can be taken in from 

 the soil, while the rest of the plant can emerge from the 

 water. This moss plant has assumed a new form. It now 

 has a distinct root, stem, and leaf, each of which has a 

 special function. The root absorbs food, the stem distri- 

 butes it, and the leaves manufacture starch from CO2 and 

 water, by the help of the sun's energy. Each of these 

 structures is exceedingly primitive in its form and func- 

 tion, but they are the foundation of the large terrestrial 

 plant of modern times. The mosses have a handicap, how- 

 ever; they can never get completely away from water. 

 They are obliged to keep within range of moisture in order 

 to liberate their free-swimming sex products. 



The fern stage is a distinct step upward. It has a 

 distinctly improved conductive system, real leaves, real 

 stem, and a real root. Its reproduction system is modified. 

 The asexual cycle has attained great proportions, as seen 

 in the tree ferns, and the sexual cycle, though not less 

 important, has been reduced to a tiny moisture loving 

 plant, small enough to allow the male sex cells to swim 

 to the female cells, which have lost their power to swim 

 about and leave the plant that produced them since the 

 early moss stage. 



The problem of getting away from a method of repro- 

 duction which is dependent on water as a medium of dis- 

 persal for its sex products is extremely difficult. The solu- 

 tion is at least attained in the production of a seed. At 

 first the seed is naked and extremely primitive (Gymno- 

 sperms — palms, ginkgos and pines). In a still more ad- 

 vanced stage the seeds are more evolved and well pro- 

 tected. (Angiosperms — hardwoods, shrubs and flowering 

 plants). The flower which produces the seed, together with 

 the seed, contain the key to the success of the modern 

 plants. The plant which started as a ven' lowly, rather 

 undecided organism, has now achieved the ability to live 

 entirely on land. The sexual and asexual cycles have been 

 neatly tucked away in the small compass of the flower and 

 seed. This remarkable evolutionary achievement has al- 

 lowed the plant to devote a great deal of energy to the 

 formation of complicated independent conducting systems 

 for both manufactured foods and (or potential food ele- 

 ments and water. 



The support of larger and larger structure has been taken 

 care of in the formation of great bundles of framework 



whose function is to maintain 

 the rigidity of the plant, 

 holding it upright. The cells 

 devoting themselves to this 

 task are a magnificent engin- 

 eering achievement. 



The most imposin.g of the 

 modern seed plants to-day are 

 certainly the big tree and the 

 giant sequoia. But for the 

 highest attainment in evolution- 

 ary complexity, as well as in 

 numbers now populating the 

 earth, we must look to the 

 less spectacular sunflower 

 group (asters, goldenrods, dan- 

 delions, etc.). Our original one- 

 celled plant reaches the pin- 

 nacle of achievement in a beau- 

 Sea Palm with Holdlasts" tiful chrysanthemum. 



