DIV. n 



SPERMATOPHYTA 



561 



from the older to the younger flowers is effected by small insects. The flowers at 



first stand upright with a widely-opened mouth (Fig. 534 I), and in this condition 



the insects can easily push past the 



downwardly - directed hairs which 



clothe the tubular portion of the corolla 



and reach the dilated portion below. 



Their exit is, however, prevented by 



the hairs until the stigma has withered 



and the anthers have shed their pollen. 



When this has taken place (Fig. 534 II) 



the hairs dry up, and the insects 



covered with pollen can make their 



way out and convey the pollen to the 



receptive stigmas of younger flowers. 



All these varied and often highly 

 specialised arrangements to ensure 

 crossing indicate a tendency to favour 

 the union of sexual cells which differ 

 in their hereditary characters more 

 widely from one another than would 

 be the case if derived from the same 

 flower. The progeny from allogamous 

 fertilisation tend to be stronger than 

 from autogamous fertilisation. 



In certain plants in addition to the 

 large CHASMOGAMOUS flowers, pollin- 

 ated by wind or insects, small incon- 

 spicuous flowers occur which never open 

 and only serve for self- fertilisation ; 

 these CLEISTOGAMOUS flowers ( 6 ) afford 

 a further means of propagating the plant, while the plants have the opportunity 

 of occasional cross-pollination owing to the presence of the large chasmogamous 

 flowers. Cleistogamy is of frequent or regular occurrence in species of Impatiens, 

 Viola, Lamium, and Stellaria, in Specularia perfoliata, Juncus bufonius, etc. 

 Polycarpon tetraphyllum has only cleistogamous flowers. 



Development of the Sexual Generation in the Phanerogams 



A. In the Gymnosperms (") a prothallium consisting of a few cells 

 is formed on the germination of the MiCROSPORE. This lies within the 

 large cell, which will later give rise to the pollen-tube, closely 

 applied to the cell wall ; the nucleus of this cell is marked k in Fig. 

 536 A, The first-formed cell (p) corresponds to the vegetative cells of 

 the prothallium. The SPERMATOGENOUS CELL (sp), which is cut off 

 last, divides later into the MOTHER CELL OF THE ANTHERIDIUM (Fig. 

 536 B, m), and a STERILE SISTER CELL (s) adjoining the prothallial 

 cell. It is by the breaking down or the separation of the sterile 

 sister cell that the antheridial mother cell becomes free to pass into 

 the pollen-tube. There, or before its separation, it divides into two 

 daughter cells ; these are the GENERATIVE CELLS or MALE SEXUAL CELLS. 



20 



FIG. 534. Flowers of Aristolochia dematitis cut 

 through longitudinally. /, Young flower in which 

 the stigma (N) is receptive and the stamens (S) 

 have not yet opened. II, Older flower with the 

 stamens opened, the stigma withered, and the 

 hairs on the corolla dried up. (x 2. After 

 F. NOLL.) 



