POLLEN. 103 



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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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

 turbance or breath of air carries it away in clouds. 



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

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

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

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

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

 The little projections of the surfaces of adjacent grains interlock like the wheels of 

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

 hundreds of neighbouring pollen-grains hang together like burs. Thai such masses 

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

 hardly needs further demonstration. 



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

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

 upon. We may thus summarize the whole matter in the statement that the 

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

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

 the adhesiveness of the poUen-grains is increased. 



A.ccording as one or other of these arrangements is present or absent we find 



