746 PHYSIOLOGY. 



mately falls off, either by its own weight, or in consequence of the 

 subsequent increase of the stem upon which it is placed, or by 

 the operation of external causes. 



According to Dr. Inman, the fall of the leaf in those cases 

 where no articulation exists " is not an accidental occurrence, 

 arising from vicissitudes of temperature and the like, but a 

 regular and vital process which commences with the first forma- 

 tion of the organ, and is completed only when that is no longer 

 useful." Asa Gray thus describes it: — "The formation of the 

 articulation is a vital process, a kind of disintegration of a trans- 

 verse layer of cells, which cuts olF the petiole by a regular line, 

 in a perfectly uniform manner in each species, leaving a clean 

 scar {figs. 193 and 349 f), at the insertion. The solution of 

 continuity begins at the epidermis, where a faint line marks the 

 position of the future joint wliile the leaf is still young and 

 vigorous ; later, the line of demarcation becomes Aveli marked, 

 internally as well as externally ; the disintegrating process ad- 

 vances from witliout inwards until it reaches the woody bundles; 

 and the side next the stem, which is to form the surface of the 

 scar, has a layer of cells condensed into what appears like a pro- 

 longation of the epidermis, so that when the leaf separates," as 

 Inman says, " the tree does not suffer from the effect of an open 

 wound." Gray, then quoting Inman, adds : — " The provision for 

 the separation being once complete, it requires little to effect it; a 

 desiccation of one side of the leaf- stalk, by causing an effort of 

 torsion, will readily break through the small remains of the 

 fibro-vascular bundles ; or the increased size of the coming leaf- 

 bud will snap them ; or, if these causes are not in operation, 

 a gust of wind, a heavy shower, or even the simple weight of the 

 lamina, will be enough to disrupt the small connections and 

 send the suicidal member to the grave. Such is the history of 

 the fall of the leaf." 



8. Development of Leaves. — This subject has been already 

 alluded to. (See p. 184.) 



Having now briefly alluded to the special functions of the 

 elementary structures, and of the organs of nutrition, we pro- 

 ceed in the next place to treat of the special functions of tho 

 organs of reproduction, but those who may desire to finish the 

 description of the life of the nutritive organs, may pass at once to 

 Chapter 2, which treats of the General Physiology of the Plant. 



Section 3. — Physiology of the Organs of 

 Keproduction. 



1. Functions of Bracts and Floral Envelopes. — One of 

 the princi))al offices performed by tlicsc organs is, to jjrotect the 

 young and tender parts placed within them from injury. When 

 green, their colour is due to the presence of chlorophyll in their 



