[Chap. 11 



THE PARTS OF PLANTS 



15 



either long or short; it depends both on the kind of plant and on the en- 

 vironment. Some of the autumn flowering plants start from seeds in the 

 spring. Some grasses, clovers, primroses, and carrots start from seeds in 

 late summer and remain in the vegetative condition until the second 

 summer. The so-called century plant grows vegetatively 20 to 30 years 

 before it bears flowers, fruits, and seeds. Woody plants grow vegetatively 

 from a few to many years, and then for many succeeding years reproduce 

 during each growing season. In later chapters it will be shown how the 

 vegetative period of a plant may be lengthened or shortened b\' such 

 external factors as light, temperature, moisture, and soil salts. 



<3q 6^&j6 



Fig. 11. The duckweeds are the smallest of the flowering plants: A, Wolffia 

 Columbiana; B, Lemna minor; C, L. trisulca, and D, Spirodela pohjrhiza, natural 

 size; E, Wolffia, enlarged; F, flowers of L. trisulca on floating plants, natural size; 

 G, a flower of L. minor, much enlarged. F and G drawn from photographs by 

 W. H. Camp and L. E. Hicks. 



We have referred thus far only to the more familiar seed-bearing 

 plants, most of which have the six principal plant organs in one form or 

 another. Some seed plants lack one or more of these organs. Some of 

 the duckweeds have a small globose body with no distinction of root, 

 stem, or leaf; others have a flattened body with a well-formed simple 

 root; all of them have flowers, fruits and seeds (Fig. 11). The well- 

 known Spanish "moss" of the South lacks roots. Many cacti are notable 

 for the absence of leaves (Fig. 12). In some plants, such as the dandelion 

 and the common plantain, the stem is merely a small flattened cone at 

 the top of the root. The tropical parasitic plant, Rafflesia, has neither 

 roots nor leaves — just a short stem, a flower, a fruit, and seeds ( Fig. 13 ) . 



Ferns have roots, stems, and leaves but no flowers, fruits, or seeds. 



