The Profound Effect on the Structure of Plants 67 



gree that their original nature would hardly be suspected were it 

 not for the existence of intermediate stages. And not only that, 

 but conversely, substantially all of the structures performing 

 remarkable or unusual functions and displaying remarkable forms, 

 are simply transformations of the three primary parts, leaf, stem 

 and root. This subject of the formation of all the special organs of 

 plants out of leaf, stem, and root, (a typical example, by the way, 

 of morphological study,) we must now proceed to consider. 



The particular structures performing definite functions in typical 

 plants, other than ordinary leaf, stem, and root, are the following : 



Bud coverings, or scales, give needed protection to living buds 

 over winter. Adaptively to this function, they are small, con- 

 caved, thick, corky, brown, and often resinous, as the large winter 

 buds of any cormnon trees will illustrate. 

 Bud scales are transformed leaves, usually 

 leaf -blades, but in some plants (e. g., the 

 Horse Chestnut) are petioles, the blades 

 being suppressed, while in others they are 

 stipules, as shows very beautifully in the 

 Tulip Tree (figiu'e 17.) 



Tendrils, or similar parts, enable slender 

 plants to cling to a support and thus mount 

 upward tow^ards the light. Adaptively to 

 this function they are slender, tough, cy- 

 lindrical, or cord-like structures, endowed 

 with remarkable powers (to be later con- 

 sidered in the chapter on Irritability), of 

 reaching out for a support, taking a firm hold thereon, and sub- 

 sequently shortening and toughening their structure (figure 85). 

 The best tendrils, like those of the Passion Vine or the Grape, are 

 transformed stems, issuing from buds precisely as branches do. 

 Others are transformed leaf-blades, as in the curious Lathyrus 

 Aphaca (figure 18), or a part thereof, as in Vetches, or Bignonia; 

 or are stipules, as in the Wild Smilax, or merely the petiole 



Fig. 17. — The stipular bud 

 coverings of the Tulip Tree; 

 one-third natural size. 



