THE COURSE OF THE POLLEN-TUBE. 



411 



the ovary, sometimes towards the central column, as in the Star of Bethlehem 

 (Ornithogalum, figs. 315 s.*. 6); whilst frequently the ovule is inverted so that the 

 micropyle faces the base of the ovary (cf. fig. 211 », p. 79). ^ince, in the majority 

 of plants, several ovules are contained in a single ovary and each is fertilized by a 

 separate pollen-tube, we find a number of tubes traversing the style, and, on their 

 entering the cavity of the ovary, diverging to the several ovules. One would 

 expect now to find this portion of the route to be followed by the pollen-tubes well 



Pig. 314. — The course of the pollen-tubes in a Kock-rose (H.^ianthemum marifolium). 



I A single flower, natural size. 2 A flower, stripped of its sepals and petals, showing stigma and style and ovary in longitudinal 

 section ; the pollen-tubes are seen passing down the style to the ovary and there going one to each ovule (the tubes are 

 indicated erroneously as going direct to the micropyles ; actually they follow a more devious course, first down the inside 

 wall of the ovary and then up to the micropyles); x22. a Stigma and upper portion of style in longitudinal section; shows 

 pollen-grains attached to the stigmatic papillce and tubes penetrating the tissues ; x 55. ^ A dry pollen-grain; x 300. SA 

 moistened pollen-grain developing its tube; x 300. ^ An ovule— which in this plant is of rather unusual form, the 

 micropyle being at the end of the ovule distant from the point of attachment; x50. 



indicated, either mechanically as by furrows and grooves, or by lines of secretory 

 and nutrient cells leading to the micropyle. This, however, seems to be very rarely 

 the case. As a rule such obvious guiding mechanisms are wanting. The pollen- 

 tubes creep along the inner wall of the ovary to the places where the ovules are 

 borne and then turn up and enter the micropyles, one pollen-tube to each ovule. 

 The adjacent figure 314 ^ shows for a Rock-rose (Helianthemum) the whole course 

 of the tubes from the stigma to the micropyles. As the tubes enter the cavity of 

 the ovary they diverge and pass one to each ovule. A slight error, however, has 

 crept- into the figure in question in that the tubes are represented as passing direct 



