POLLEN. 103 



otoplasm within, destined for fertilization, takes up water from the environment 

 iry quickly and energetically. In consequence it swells rapidly, and must have 

 L inclosing wall which will not impede its rapid stretching. For this purpose the 

 in places and folds are admirably suited. Through them fluids readily pass to 

 .e interior, and simultaneously the grooves (previously folded inwards) become 

 flated, and the pollen-grains come to occupy two to four times the space they 

 •eviously did. The thicker portions, saturated with oil, play a purely passive role 



these events. Water cannot enter by these parts, nor do they stretch with 

 le swelling up inside. Later, when the intine has grown out and assumed the 

 irm of a tube^ the outer wall is not essentially altered. The thin spots have been 

 iptured, and where lids are present, they are raised; the protoplast, enveloped 

 1 the tube-like intine, vacates the extine by one of the thin spots, much as a 

 3rminating embryo does its seed-coat. Just as it is of advantage in germination 

 >T the seed-coat to be fixed on the substratum, whilst the young plant gets a good 

 Did of the ground, so here it is of value to the young pollen-tube as it quits the 

 dine of the pollen-grain that the coat should be fixed firmly; for this purpose the 

 irious ridges, teeth, and spines possess a high significance, serving as a means of 

 ichoring the pollen-grain whilst the pollen-tube is being formed. 



But the most important service rendered by the sculpturings and inequalities of 

 le walls consists in the fact that thereby considerable quantities of pollen-grains 

 •e enabled to cohere in crumbling masses to the slits of the opened anthers, and 

 i become attached to insects and other animals visiting the flowers for food. Con- 

 asting with this clinging pollen is the already-mentioned dusty pollen, with 

 aooth and non-adhesive surface. Dusty pollen does not cohere in clusters, nor 

 )es it readily attach itself to foreign bodies. On the other hand, the least dis- 

 irbance or breath of air carries it away in clouds. 



It is sufficiently obvious that globular or ellipsoidal pollen-grains with smooth 

 irfaces will be distributed in the form of dust more readily than grains possessing 

 I angular or crystalline form. The former have a smaller surface of contact 

 :an the latter. When the surface is, in addition, variously sculptured and raised 

 to folds and inequalities, the points of contact are of course enormously increased- 

 ae little projections of the surfaces of adjacent grains interlock hke the wheels of 

 watch; longer ones become entwined like fingers; thus it comes to pass that 

 mdreds of neighbouring pollen-grains hang together like burs. That such masses 

 ill readily attach themselves to the hairs, bristles, probosces, and legs of insects 

 i,rdly needs further demonstration. 



This capacity for clinging is much increased when the surfaces of the grains are 

 turated with oil. The sticky property of the viscin has been already enlarged 

 )on. We may thus summarize the whole matter in the statement that the 

 ystalline forms, the various sculpturings, spines, and other projections, as well as 

 e presence of oil and viscin on the surface are arrangements in virtue of which 

 e adhesiveness of the poUen-grains is increased. 



According as one or other of these arrangements is present or absent we find 



