ORIGIN OF CONDUCTING TISSUES. 167 



till from tlie twelfth to the fifteenth day that some of them 

 arrive at the base of the ovarj. Before the sexual apparatus 

 is complete, the extremities of the pollen-tubes separate from 

 the mass of tubes overlying the conducting tissue, twist in 

 various directions, scrambling over the placentary lobes and 

 their ramifications, and so approach nearer and nearer to the 

 ovular fucicles ; but they only penetrate the micropyles, 

 after the formation of the sexual apparatus. It is supposed 

 by Strasburger that the synergid^ expel a liquid destined 

 to guide the pollen-tube to the embryo-sac ; others think their 

 function is to aid in the solution of tissues for nourishment. 

 In Vanilla, for example, the upper part of the embryo-sac 

 is absorbed where occupied by the synergidae, and is then 

 covered by the elongated border of the primine. M. Guignard, 

 however, adds : — " II est possible qu'il soit attire par un 

 liquide expulse par les synergides,* comme le pense M. Stras- 

 burger, ou bien aussi, comme je crois I'avoir constate, par 

 I'etat special de la couche superficielle des membranes cellu- 

 laires da bord interne du tegument." f 



On the action of the pollen-tubes M. Guignard writes as 

 follows: — "Au contact des faisceaux polliniques, le tissu 

 conducteur offre un contenu riche en sucre reducteur ; I'ami- 

 don, dans le cas actnel, ne se trouve qu'au voisinage et du 

 cote externe des faisceaux libero-ligneux des parois ovari- 

 ennes. Outre le pouvoir d'attaquer la substance amylacee et 

 d'intervertir la saccharose, comme I'ont montre tout recem- 

 ment M. Yan Tieghem, J et M. Strasburger, § les tubes polli- 

 niques peuvent aussi, a I'aide des ferments qu'ils contiennent, 



* Synergidce is better, being nearer Sunergatai. 

 t L.c, p. 209. 



X Sur V Inversion du Sucre de Canne jpar le Pollen, Bull. Soc. Bot. de 

 France, 1886. 



§ Ueher Fremdartige Bestauhung, Pringsh. Jahrb., vol. xvii. 



