June 2, 1906 



HORTICULTURE 



709 



v Fertilization of Spruce and Willow 



Recently, when the willows were in 

 bloom, I had occasion to make certain 

 studies of the flower and obtained 

 drawings which may possibly prove 

 interesting, because they tell very well 

 the story of fertilization, making clear 

 a more or less hidden and secret pas- 

 sage in the life history of these trees. 

 The willow is one of the very first 

 members of the awakening plant 

 world to put forth flowers. The bees, 

 bestirring themselves after a winter's 

 rest, go from tree to tree in quest of 

 honey, and as they clamber over the 

 staminate catkins, become dusted with 

 the pollen, which is later brushed off 

 upon the stigmas of the pistillate cat- 

 kins. This is only the beginning of 

 the fertilizing process. How the pollen 

 brings about the fertility of the seed 

 is more mysterious. 



As I write the spruce is in full 

 flower, and the young cones are beau- 

 tiful indeed, in red, orange and purple 

 against the dark needle-foliage. Vari- 

 ous species of pines, the hackmatack, 

 and the hemlock are either in bloom 

 or soon will come out. The observant 

 wayfarer passing through the woods 

 in May or June will note drifting 

 trains and clouds of coniferous pollen. 

 "The immense abundance of pollen, 

 its lightness, and its free and far dif- 

 fusion through the air in Pines, Firs, 

 Taxodium, and other Coniferae, are 

 familiar. Their pollen fills the air of 

 a forest during an thesis; and the 

 showers of sulphur, popularly so- 

 called, the yellow powder which after 



^ 



Fig. 1. Flowers of the white spruce, 

 slightly enlarged. Above, the female 

 cone; below, 2 male cones. 



a transient shower, accumulates as a 

 scum on the surface of water several 

 or many miles from the nearest 

 source, testifies to these particulars" 

 (Gray). 



The conifers, therefore, depend for 

 pollen-transport upon the winds 

 rather than on insects. It is interest- 

 ing to see that the lowest Dicotyle- 

 dons — lowest and therefore nearest to 

 the Gynosperms, if, indeed, the Angio- 

 sperms are related to the Gymno- 

 sperms at all — are in general wind- 

 pollinated also; the willow being ex- 

 ceptional among the primitive, cat- 

 kin-bearing trees. Walnuts, birches, 

 hazels, hornbeans. alders. oaks, 

 beeches, chestnuts, etc. — all low in the 

 scale of Angiosperms — are wind-polli- 

 nated. As we ascend In the series, 

 the plaiit shortly is seen to depend on 

 insect aid and the flower becomes 

 more and more adapted to the attrac- 

 tion and reception of these visitors. 



The spruce has been an object of 

 study also, and the drawings supple- 

 ment those of the willow; for to- 

 gether the two plants fairly well re- 



present the whole vast group of seed 

 plants in its two grand divisions, the 

 one with openly exposed seeds, the 

 other with seeds in a closed cavity. 

 To take the conifer first, for that is 



B 



Fig. 2. A scale from female cone, with 

 ovules (ov); B, 2 stamens with pollen 

 sacs (p s), the pollen failing from one; 

 these from the male flower. All mag- 

 nified. 



the lower, the rounded scale in figure 

 2 was taken from the female flower 

 or cone, the uppermost in figure 1. 

 A pair of young seeds on the upper 

 side are all ready for fertilization. 

 In fact these egg-shaped bodies are 

 scarcely seeds yet, 

 merely ovules, lying 

 openly on the upper 

 side of the scale wait- 

 ing for the coming of 

 the wind-borne pollen. 

 In due time this falls 

 from the stamens of 

 the male cone (fig. 1, 

 lower cones), and after 

 a longer or shorter 

 aerial voyage lodges in 

 the openings between 

 the scales of the female cone. 

 One of the grains as viewed 

 through a strong magnifier is seen 

 in the third figure. On apply- 

 ing the micro-metric scale to the 

 grain I find that its diameter is about 

 one three hundredth of an inch. Its 

 lightness and buoyancy are increased 

 by two air sacs (g, s). The main body 

 of the grain is filled with soft living 

 substance, its organs dimly visible in 



the pore and into a chamber of an 

 ovule; at any rate it finally lodges at 

 the point where three pollen grains 

 were actually discovered in one of my 

 sections, as shown at g in figure 4. 

 The pollen grain is now in contact 

 with the kernel of the ovule and I 

 presume is held by some moisture of 

 the walls. In a short time, that is 

 within a day or less, as I conclude 

 from the behavior of the cones, the 

 grain germinates. A tubular exten- 

 sion of the body-wall appears (pt), 

 which plunges into the tissue of the 

 nucellus, like a root into soil, and 

 grows toward a sac, the so-called em- 

 bryo-sac, shown at s in figure 4. Some 

 of the living substance from the pollen 

 grain follows down the interior of this 

 tube, in particular a rounded body of 

 very small size but of very definite 

 structure and properties, bringing 

 with it, in fact, all the characters of 

 the spruce tree from which the pollen 

 came. How such a mere speck of 

 matter can be freighted with the in- 

 finite variety of inheritance which ob- 

 servation and experiment shows that 

 it must convey, is one of the greatest 

 marvels of organic life. Yet in this 

 minute sphere within the advancing 

 pollen tube lie the inner determinants 

 of all the features of the species, — 

 the general stature and form of the 

 whole tree, the color and texture of 

 bark, the arrangement of the branches 

 and leaves, the shape of the latter, the 

 form, color and arrangement of the 

 cone-scales, the shape of future pollen 

 grains, the character of the wood, the 



Fig. 3. A single pollen grain of spruce, 

 highly magnified; s, s, the air sacs for 

 buoying the grain up; b, the body. 



the interior. The newly arrived pol- 

 len grain rolls down between the 

 scales, taking a somewhat circuitous 

 course, and perhaps it is the impetus 

 thus gained which carries it through 



Fig. 4. Scale and ovule cut so as to ex- 

 pose the interior of the ovule; s, em- 

 bryo Bac; pt, pollen tube; g, pollen 

 grains; magnified. 



shape of wood-fibers, and the quality 

 of the resin, and so on down to all the 

 details of structure, gross or minute, 

 and of habits, physiology, adaptation 

 to particular habitats, time of flower- 

 ing, longevity of the tree, etc. In 

 short the constitution of the future 

 spruce tree is determined by this lit- 

 tle organism — if the word may be ap- 

 plied to a complex part as well as a 

 whole plant — in conjunction with a 

 similar body in the sac toward which 

 the tube is now working its way. The 

 tube finally arriving at its destina- 

 tion, the end wall is dissolved or 

 burst, and the fertilizing body enters 

 the sac, where it unites and fuses with 

 the one already present, the first de- 

 finite structure of a new spruce tree 

 being the result. This is fertilization. 

 From this beginning the tissues of 

 the embryo arise. 



The willow has its ovules entirely 

 shut in and the pollen shut out. The 

 pollen tube, therefore, which arises 

 from the pollen grain, must travel a 

 much longer distance than that of the 

 spruce. The figure (fig. 6) shows the 



