This spot, - the stigmatic surface, - is the goal of the microspores. It is very various in its appearance in different flowers. Sometimes it is a little knob, sometimes a small point, sometimes, as in this crocus, it spreads into many rays like a star. In many flowers it is covered with short hairs, or with minute knobs, among which pollen-grains may be caught and held fast. In the orchids it is just a little surface of bare tissue. But, whatever is its outward semblance, Nature has prepared it to receive pollen by moistening it with a sugary fluid, so that any grains which touch it may adhere, and may germinate upon it.
Directly a speck of the life-giving dust settles down on the stigmatic surface it begins to do its appointed work there. In most instances the thin inner coat of the little bag swells up at one place into a hump, which thrusts itself through one of the holes in the outer case, or pushes off one of the lids, or, it may be, forces its way outward through a thin spot (Fig. 3b). The hump grows bigger, becoming a sac, and, at last, a tube, which, in some flowers, attains a length of several inches. This tube grows downward into the substance of the pistil, much as a strong rootlet burrows into rich light soil.
All is ready for its reception. The part of the pistil which it must penetrate is never filled with anything more substantial than a loose mass of large cells, called "conducting tissue," and, in some few species of blossom, it is empty. So in due time the end of the pollen-tube reaches one of the baby seeds in the pistil's base, and enters it by a minute orifice in the seed-coat.
Fig. 3b. - Pollen-grains of the European hazel or filbert (Corylus Avellana) putting forth their pollen-tubes.
Inside the baby seed is another little globe or sac filled with colorless jelly - the "macrospore" or embryo-sac. The pollen-tube pushes its way downward till it touches and pierces this little globe. Then part of the drop of jelly which has filled the pollen-grain or microspore enters the macrospore and fuses with its jelly, and when this union takes place the purpose for which the blossom blew has been achieved. From the fusion of microspore and macrospore comes life, or rather the possibility of life, for from their united substance Nature begins to mould and build a tiny plant within the young seed.
The time which elapses between the first touch of the microspore upon the stigmatic surface and the quickening of the seed that is to be, varies greatly in flowers of different species. The pollen-tube of the crocus takes from one to three days in finding its way to the macrospore. But this is not because the crocus pistil is long, for in the great night-blooming cereus, which has a pistil nine inches in length, the pollen-tube penetrates to the macrospore in a few hours, while in some flowers, as in certain varieties of orchid, weeks elapse while the tube is descending a very short distance.
Each macrospore can be vitalized by the contents of one single tube, so but one microspore is necessary to the development of a seed.
But Nature provides the golden dust in lavish profusion. It has been estimated that twenty thousand grains are contained in one single stamen of a peony, and some stamens yield the vitalizing powder in even greater abundance.
This is because Nature must provide microspores enough to meet the needs of all the macrospores in all the flowers that blow, after an enormous amount of the precious powder has been wasted.
Some blows away, some is washed earthward by rain or dew, some is eaten by ants and other crawling intruders, much is gathered by the bees, to be made into "bee-bread," and many grains are dropped by flying insects, before the pistil of a sister blossom has been reached.
The use of pollen in the floral economy was suspected, - at least in the case of certain blossoms, - even in classic times. And the fact that the pollen-grain must give of its substance to the pistil before the seed can be vitalized has been known for two centuries. But only in recent times have Nature-students made a discovery which casts a flood of light upon the mysteries of the flowers, - and it is this: The macrospore in most cases is vitalized not by the pollen of the flower in which it is formed, but by the pollen from some other flower of the same species.
And even those flowers which can make shift to get along with home-made pollen achieve better results with the imported article.
Thus the pistil of the crocus will form larger and stronger seeds if it can get pollen from a sister blossom, or, better still, from another crocus plant altogether. So the flowers wish to send the yellow powder about, from one to another, for their mutual benefit, and the bee behaves as if she had been taken into their confidence. She has flown out of our yellow crocus now, as dusty as a miller, and has gone droning into another one, which is growing on the opposite side of the garden walk. As she reaches down into the bottom of its chalice, for the sweets she hopes to find there, some grains of the pollen she has brought in with her will be rubbed off her velvet jacket onto the waiting pistil.
Crocus number two accepts this unintentional donation with pleasure, pays for it with a drop of nectar, and gives also a sprinkling of pollen from her own stamens. The bee, carrying the powdered gold which has just been bestowed upon her, flies off to make a call upon a third crocus, and when she departs she leaves some of her dusty load behind her, as a souvenir of her visit.
So each crocus "sets" its seed by aid of pollen brought from another flower.
Each flower has gratified its preference for yellow dust of foreign manufacture, and has received enough of the imported article for her dainty uses, and each has sent the pollen of her own making to the exact spot "where it will do the most good." The bee meantime has been entertained everywhere with pretty shows and luxurious fare, and she is another well-satisfied member of the mutual benefit society.