INTRODUCTION. 



37 



light ; the exhalation being very slight during the night. When the fluid 

 thus escaping is greater than that absorbed by the roots, the leaves droop and 

 wither; in general, however, these two functions of exhalation and absorp- 

 tion are in direct proportion to each other, and hence the sap ascends with a 

 rapidity corresponding to the activity of the exhalation. 



The immediate cause of this ascent has not been fully ascertained, and 

 many theories have been devised to account for it; as the action of heat, 

 capillary attraction, vital contractility of the vessels and cells, electric power 

 and endosmosis ; all these probably have their influence, though the latter 

 process combined with capillary attraction and the exhalation from the leaves 

 will explain most of the facts. When the sap is consumed in a proportion 

 equivalent to the absorption of fluids, no accumulation of it, to any extent, can 

 occur ; but when the leaves fall off, the roots still absorb to a certain degree, 

 and hence the stem becomes gorged with fluid ; this gradually undergoes a 

 change, and forms deposits in the cells and tubes of the wood. In the spring, 

 also, absorption commencing before the formation of leaves permits a due 

 exhalation, the trunk again is loaded with a superabundance of sap. 



When the sap is thus concentrated by evaporation, and exposed to the 

 action of the air in the tissues of the leaf, it undergoes certain changes, and 

 forms new and peculiar products, when it is called the proper juice. This is 

 collected by a system of vessels on the under surface of the leaves, which 

 communicate with the bark; these are principally of the kind previously 

 spoken of, as vessels of the latex. Hence the peculiar products of woody 

 exogenous plants are found in the bark or wood, where they accumulate 

 from year to year. In perennial herbs they are found in the root or sub- 

 terranean modifications of the stem ; whilst in annuals they are diffused 

 through all parts of the plant. 



The roots have hitherto been spoken of as the sole organs of absorption ; 

 but this process is likewise carried on by the leaves, and in some tribes of 

 plants they are the principal organs of nutrition, as in the Cacti, and in many 

 of the parasitic Orchideee. This absorption of fluid by the leaves takes place 

 mainly through the cuticle, and especially by means of the downy hairs 

 which act like root fibres. 



Fecundation of Plants. 



In flowering plants, fecundation takes place by the ac- 

 tion of the pollen of the anthers on the pistil. These mi- 

 nute cells or grains, when from any cause they come in 

 contact with the stigma of the pistil, adhere to it by the glu- 

 tinous secretion it exudes, and in a short time change in a 

 remarkable manner; the outer coat of the pollen-cell open- 

 ing and allowing the inner coat to protrude in the form of a 

 tube of extreme tenuity, filled with the fluid and granular 

 matter already spoken of; this passes between the cells of 

 the stigma, and passes down through the loose texture of 

 the style, until it reaches the ovary ; whilst this is going on, 

 a cavity or sac, called the sac of the amnios, filled with 

 fluid, makes its appearance in the nucleus of the ovule; 

 and the foramen being brought in contact with the pla- 

 centa, or in orthotropous ovules, carried up to the summit 

 of the cell, a pollen-tube enters the orifice and the granules 

 thus reach the nucleus. What further occurs is not known, 

 but it is ascertained that in a short time the embryo ap- Section of style of An- 



A i • ,• * ' tirrhinum, showing the 



pears in the amniotic sac. passage of poiien-tubes. 



