Part 196. Cryptograms And Phenogams - Plant Life-Histories. Part 3

Fig. 382.-Climbing Buckwheat (Polygonum Convolvulus, Buckwheat Family, Polygonacece). Pistil (48/1) during fertilization, cut vertically to show stalk-like base (fs) of ovary; stalk of ovule (fu); end of stalk (cha); the nucellus (nu), the micropyle, or opening to the nucellus (mi); the inner integument (ii); the outer integument (ie); the embryo-sac (e); nucleus of the embryo-sac (ek); the egg-apparatus (ei) consisting of three cells, the lower one being the egg-cell and the two others companion cells which are thought to represent rudimentary archegonia; prothallial cells (an); style (g); stigma (n); pollen grains (p); and pollen-tubes (ps). A pollen grain falling upon the stigma produces a tube which grows down through the style, enters the micropyle, and penetrates to the egg-cell; here it discharges its (male) nucleus which fuses with the nucleus of the egg-cell and from this union an embryo arises. (Strasburger.)-The plant is an annual vine resembling buckwheat but with greenish flowers; native to Europe but a common weed in America.

Fig. 383.-Shepherd's Purse (Capsella Bursa-pastoris, Mustard Family, Cruciferoe). Development of the embryo. A-D, successive stages, much magnified, showing the suspensor (et), lower end of embryo (h), cotyledons (c), and the stem-tip (p) from which the plumule arises. (Hanstein.)-The plant is annual, about 50 cm. tall, with small white flowers and dry fruit; native of Europe; common as a weed in America.

While it is not altogether clear how closed ovaries evolved from open carpels, the change may well have taken place as a result of the peculiar inrolling of the young leaf-lobes which primitive gymnosperms are supposed to have inherited from ferns. If ovules should form on such lobes while still inrolled, and the lobes should coalesce, the carpel would be angiospermic. However it happened, many highly important consequences of this advance are apparent. Most obvious is the greater protection of the offspring during the period of their dependence.

There is also an enormous gain in possibilities for securing an advantageous cross. Thus a moist, projecting stigma becomes a target more likely to be hit by wind-carried pollen grains and more likely to insure their prompt germination, than the micropyle of a naked ovule. Expansion or branching of the stigma, such as we find to be characteristic of wind-pollinated angiosperms, shows how well this new organ lends itself to increasing the chances of pollination and at the same time favors economy in the amount of pollen produced. See Figs. 2-15, 27, 36, 74 B, C, 109 C, D, 124 B, 153, 159, 165 II, 171 II, 243, 248, 254, 256 B, 257, 267 D, etc. A still further advance in these directions came when insects and other animals were attracted to the flowers, and their services thus secured as carriers of pollen. The attractive odors, bright colors, and alluring sweets, together with the marvelous arrangements of the various floral organs, present modifications of endless variety and offer one of the most fascinating fields in the whole range of botanical study. See Figs. 22, 39 II, 48, 57, 59 77, 80, 91-100, 106, 133, 139, 145, 148, 156, 163, 164, 168 II, 172, 178, 187, 188, 189, 192, 217, 251, 275, 276, 282-293, 299, etc.

This elaborate modification of stem-parts and leaf-parts cooperating with pollen-sacs and ovules to form what we call a flower brings with it the further possibility of utilizing these accessory organs, or certain parts of them, to further protect the embryo and provide for its surer and wider dispersal. Hence arise protective shells (Figs. 23-36) and various aids to dissemination, including wing-like or plume-like appendages (Figs. 55, 59, 76, 159, 197, 215, 248, etc.), for catching the wind; elastic springs for propelling the seeds to a distance (Figs. 164, 165), and succulent parts (Figs. 88-110) which being attractive food, lead animals to swallow indigestible seeds and transport them often to enormous distances. Thus, it is in plants which form seeds within a case that we find the most perfect provision for the welfare of offspring; and it is doubtless because of this provision that angiosperms are the dominating plants of to-day.

Fig. 384.-Diagrams illustrating the supposed evolution of flowering plants through various stages from algae. The heavy, branching line above stands for the ancestral line of the angiosperms, from which have come off also those main branches of more primitive plants that have representatives now living. Order of time is shown by distance above the base; increasing complexity of structure by distance toward the right. Below each branch is a diagram outlining the life-history of a typical example of the group named above it. In these diagrams by following the dotted lines in the directions indicated by the arrow-points, the successive stages of each life-cycle will be passed through in their natural order. Each cycle begins below with a single cell, and is complete when a similar initial cell is produced. In the different types corresponding stages of development are placed at about the same height, so as to come in nearly the same horizontal line, thus illustrating the theory of "recapitulation of phylogeny by ontogeny." It should be noted that the view here suggested does not imply that any living form has descended from remote ancestors closely similar to any form now living. On the contrary, it aims to express the idea that modern angiosperms evolved from extinct ancestors of modern cycads (very different from either living cycads or angiosperms) and that these common ancestors of the cycads and angiosperms were descended from extinct ancestors which gave rise also to modern ferns (although they themselves were doubtless very unlike any ferns of to-day); and so on back to the earliest forms of life. As we try to trace the line back beyond the ancestral ferns our conclusions must be more and more uncertain for lack of fossil remains to compare with living structures. (Original.)