THE LIFE -HISTORY OF PLANTS. 



19] 



the stigma this is evidenced by the exudation from 

 its surface of a sticky liquid. In some cases there 

 are special structm-al contrivances connected with 

 the stigma, which entrap the pollen, prevent its dis- 

 persal, and tend to secure its germination. 



Germination of th.e Pollen. — The growth of 

 pollen is in this wise : — The pollen-cells have a cellu- 

 lose coating overlain by a structureless cuticle, and 

 containing a quantity of protoplasm and one or more 



Fig. 83.— Stigma of Datura, covered with Pollen, 

 grains (magnified). 



nuclei. They are in fact fully equipped cells. 

 When the grains or cells fall on the moist stigma their 

 cuticle cracks, and allows the cell to protrude and 

 lengthen in the form of a tube (Fig. 82). Very 

 often indeed there are special cracks or apertures 

 through which the tube passes. At any rate this 

 tube gTows, and lengthening as it grows, pushes its 

 way between the ceUs of the stigma and the style 

 (Figs. 83 — 85). These cells are so constructed 

 as to allow it to derive food -supplies from them, 

 and are so arranged as to facilitate its passage. In 

 this manner, after a lapse of hours, days, or months, 

 m different plants, it eventually comes in contact 

 with the ovules, one or more tubes to each ovule 

 (Fig. 85). 



The Ovule. — Within the cavity or cavities of 

 the ovary are contained one or more small whitish 

 bodies, visible to the naked eye when the ovary is 

 cut across or downwards. These are the rudimentary 

 seeds, destined to be the nursing mothers of the 

 fertilised egg or embryo plant. The ovary therefore 

 is the direct precursor of the fruit or seed vessel; 

 the ovules, of the seeds ; the germ within the seed, 

 of the embryo or seedling plant. 



The number, shape, size, mode of attachment of 



Fig. St.— Pollen-tubes of Datura piercing Conducting 

 Tissue (highly magnified). 



ovules, vary much in different plants ; but these 

 points, valuable as they are for purposes of dis- 

 criminating one plant, or one grouj) of plants, from 

 another, are of little value for our present purpose. 

 The internal variations in structure are equally of 

 little importance for this point of view. Suffice it 

 here to say, that in general ovules consist of a central 

 body called the "nucellus," surrounded by one or by 

 two sheaths, called the " coats of the ovules," which 

 grow up from below, and leave at the top a small hole 

 called the "micropyle," so that the extreme top of 

 thenucellus is thus uncovered (Fig. 87). At the top 

 of the nucellus, moreover, is one cell much larger than 

 the rest, and which is called the "embryo-sac," be- 

 cause in it the embryo is eventually formed (Fig. 87). 



