10 



STRUCTURE AND PHYSIOLOGY OP THE PLANT 



Fig. 19. The gooseberry is a true fruit, 

 or ripened ovary. The remains of 

 a flower are shown at c. 



Fig. 21. Vascular bundles in 

 stem of moonseed. The 

 xylem part, with large open- 

 ings, is on the inner side, 

 the phloem on the onter 

 aide. Pith at P. 



Longevity of the plant. 



In duration, plants are of extreme types. Some kinds live only a few weeks ; some of the trees live 

 for many centuries. It is customary to classify all plants into three groups as respects duration : 

 annuals, living not more than one year from seed to seed, as the cereal 

 grains and most garden vegetables ; biennials, living two years, usually 

 perfecting seed the second year, as beets and parsnips, common mullein ; 

 perennials, living more than two years, as asparagus, alfalfa, bushes 

 and trees. These divisions are not at all exact, however. Annuals are 

 of longer or shorter life within the year, some maturing and dying 

 very quickly from the seed, as the garden cress, and others requiring 

 practically the twelvemonth. Some plants are annual because they are 

 destroyed by frost, and others because they 

 normally complete their growth : the latter, 

 of course, are the true annuals. Those that 

 would outgrow the year if they had oppor- 

 tunity have been called plur-annuals : they 

 are plants that have been taken into a 

 shorter - season year, as tomato, castor 

 bean. Plants that are annual in one region, 

 therefore, may be biennial or perennial in 

 another region. Some plants are appar- 

 ently annual although they live from year 

 to year, carrying themselves over by 

 means of bulbs or tubers, as onions and 

 potatoes : these have been called pseud- 

 annuals (false annuals). The mullein, bull 

 thistle and teasel are true biennials, part 

 of the growth occurring one year and the 

 completion of the life-cycle the second year. Certain perennials tave 

 been bred by man to be biennials, as the cabbage and probably some 

 root crops. Some of the root crops are really annual, as they complete the full cycle in one season if 

 started early, as the radish. Whether a plant is biennial is often determined by the region in which it 

 grows. There is the widest range in the length of life of perennials. Red clover is a perennial, but very 

 imperfectly so ; some forms of it thrive only two years, although they may persist longer. Most peren- 

 nial herbs are at their greatest vigor the second and third years, as the strawberry, and then gradu- 

 ally weaken, and sometimes even die before very old, new plants having been formed in the meantime. 

 Gardeners know that the best bloom with pinks and hollyhocks and many other showy perennials is 

 secured from plants that are only two or three years old. Sometimes the renewal is accomplished by 



dividing the old roots. 



Societies of plants. 



Since plants contend with each other and 

 since different kinds have been driven into 

 similar places or regions, it follows that 

 certain kinds have come to grow together, 

 forming plant societies or communities. A 

 certain set of plants live together in a 

 swamp, and another set on a hill, another 

 in a meadow, and another set in a cotton- 

 field or a corn-field. Certain plants grow 

 under or over other plants : grass and 

 bushes may grow under trees ; corn grows 

 above the pumpkins that are planted with it. 

 Fie. 22. The columnar trunk or stem of a monocotyiedonous plant, Wherever plants grow, they are in societies • 



not increasing much in diameter. Heneqnen Uaave Hgida var ^jj^t is, they grOW together for Certain rea- 

 elongata), sixth crop being cut, two outer rows of leaves cut ' j a & "-^ \jc>.iiaxu. xca 



every eight months. Yucatan. SOUS, — they are adapted to each other or to 



Fig. 20. In the chestnut, the nuts are 

 the true fruits. They are contained 

 In a husk. 



