DIV. I 



MORPHOLOGY 



S7 



result of this the older leaves close over the growing point (Fig. 98) 

 and the younger leaf rudiments. The growing point thus becomes a 

 bud in which the delicate younger structures are protected against 

 desiccation by the older and larger, though still immature, leaves. A 

 bud is thus the young incompletely-developed end of a shoot. 



VERNATION AND AESTIVATION.* A section through a winter bud shows a 

 wonderful adaptation of the young leaves to the narrow space in which they are 

 confined. They may be so disposed that the separate leaves are spread out flat, 

 but more frequently they are folded, rolled (Fig. 103 I), or crumpled. The manner 



FIG. 101. A, Apical view of the vegetative 

 cone of Equisetum arrense. B, Optical 

 section of the same, just below the apical 

 cell ; I, lateral walls of the segments. 

 Further explanation in text, (x 240. 

 After STRASBURGER.) 



f 



FIG. 102. Median longitudinal section of the 

 vegetative cone of Hippuris imlgaris. d, Der- 

 matogen ; pr, periblem ; pi, plerome ; /, leaf- 

 rudiment. ( x 240. After STRASBCRGER.) 



in which each separate leaf is disposed in the bud is termed VERNATION. On the 

 other hand, the arrangement of the leaves in the bud with respect to one another 

 is designated AESTIVATION. In this respect the leaves are distinguished as FREE 

 when they do not touch, or VALVATE when merely touching, or IMBRICATED, in 

 which case some of the leaves are overlapped by others (Fig. 103 ). If, as 

 frequently occurs in flower -buds, the margins of the floral leaves successively 

 overlap each other in one direction, the aestivation is said to be CONTORTED. 



(/3) The Axis of the Shoot. A. External Construction. The 



active elongation of the stem begins at some distance from the grow- 

 ing point ; with this the leaves in the bud begin to separate. It is 

 characteristic of shoots, especially aerial shoots, that this elongation is 

 not limited to a short region below the bud but extends many centi- 



* [The use of these terms in the following paragraph differs from that customary in 

 England. By VERNATION is understood the arrangement of the leaves in a vegetative 

 bud as a whole. The folding of each individual leaf in the bud is termed PTTXIS. The 

 term AESTIVATION is applied to the arrangement of the parts in a flower-bud. TRANS.] 



