This internal development continues for very different 

 lengths of time in different animals. In one of the groups 

 of mammals, the marsupials, which includes the opossum 

 and kangaroo, the young are carried only for a short time. 

 They are in a very immature condition. They are then 

 placed in an outside pouch which contains the milk glands. 

 Here the young remain until they are mature enough to 

 take some care of themselves. In the higher mammals, 

 the period within the uterus varies from a few days to a 

 year or more- For the large mammals it is usually a little 

 less than a year. In these the development at birth is 

 greater than it is in the marsupials. 



5. Birth and its Changes. Birth in mammals may be 

 said to correspond to hatching in the birds. The young 

 mammal passes through all its embryonic stages in the 

 uterus and has all its principal organs leady for use when 

 born. While it is living one kind of life (that within the 

 mother) it is being made ready for a very different one. 

 The change in birth is like that in hatching, a very sudden 

 one. The change of life in the tadpole on the other hand 

 comes very gradually. 



Before birth the young are surrounded by the body of 

 the mother and have a constant temperature- At birth 

 they are brought into a very much colder and more 

 changeable temperature. Before birth all the food and 

 oxygen are taken up by the blood from the mother's blood. 

 At birth this supply ceases, and the lungs for the first 

 time are used and oxygen taken through them. Similarly 

 the digestive tract has been developed merely for the use 

 it was going to serve, not for any value it had before birth. 

 One can readily see that birth is not in any sense the 

 beginning of life. It is merely a change from one kind of 

 life on the part of the embryo to another kind. It is 

 similar to that which the toad makes when it passes from 

 living and breathing in water to living in the air. 



6. Care after Birth. It must not be thought that the 

 mammal young are, or ought to be, completely independent 

 at birth. Just as in birds, there is a period in which the 

 young demand a great deal of care from the parent. This 

 is accomplished in many ways, but the particular form of 

 care in which we are interested is that peculiar to mammals 

 and which gives them their name. This is by means of 

 the secretion known as milk- It is produced by skin 

 glands which have become highly modified and often 

 greatly enlarged. It consists of water, of salts from the 

 blood, of a form of sugar, of considerable oil which in fine 

 particles gives the milk its whiteness, and of certain 

 proteins. It is thus a complete food for the young animal. 

 From the point of view of successful reproduction, milk 

 makes it possible for more of the young to live than could 

 possibly come to maturity without it. Carrying the young 

 in the body and furnishing them with a complete food 

 when first born are two most wonderful inventions, so to 

 speak. They go far toward insuring success to the group 

 of animals that use them. 



All that has been said about the reproduction and 

 development of mammals applies to man. 



CHAPTER NINETEEN. 

 THE YOUNG OF THE HIGHER PLANTS. 

 1. Vegetative Reproduction among Plants. Already 



we have referred to the fact that many plants produce new 

 plants by buds, or tubers, or roots, or runners, and even 

 by leaves. The higher plants differ very much in respect 

 to this rough and ready kind of multiplication. Some 

 plants have none of these methods, others may use two 

 or three of them. 



2. The Older Idea of Reproduction in Plants. Until 

 recent years the reproduction of plants by seeds would 

 have been described as follows: 



"The seed contains an embryo. When the seed 

 germinates, the embryo breaks through and develops 

 roots and leaves that enable it to get its own nourishment. 

 It now grows until it becomes mature. The mature plant 

 forms flowers which are its reproductive organs. Each 

 complete flower, in addition to the protective and colored 

 parts (sepals and petals), contains both male and female 

 organs. The stamens are male organs and the pistils are 

 female organs The pollen, arising from the stamen, is the 

 male cell and corresponds to the sperm, but has in itself 

 no power of motion. The ovary develops an egg cell deep 

 in each ovule. When the pollen is carried to the stigma 

 and the pollen tube grows down into the ovule and unites 

 with the egg, the male cell is fertilizing the female cell." 



This is not the correct interpretation of sex reproduction 

 in plants, although it is the way in which many people 

 understand it. There is enough of fact and enough of 

 fiction in this interpretation to make it necessary to give 

 the process in a little more detail. 



3. The Modern View of Reproduction in Flowering 

 Plants. Botanists now describe this process as follows: 



"The plant develops from the embryo in the seed and 

 presently produces flowers. Flowers are really clusters 

 of special leaves; sepals, petals, stamens, and pistils being 

 each a set of these special leaves about the end of the stem. 

 The stamens and pistils are like the leaves of the fern 

 plant in that they bear spores. The other leaves do not. 

 The stamen is not a male organ; it is a spore-bearing leaf 

 The whole plant is therefore a sporophyte. The pollen 

 is not a male cell; it is, at first, just a spore. The pistil is 

 not a female organ; it is also a spore-bearing leaf, and 

 deep in each ovule at least one large spore is borne. The 

 spores (pollen) borne by the stamens are smaller spores 

 than those borne by the pistil. The spores in the 

 stamen escape; those in the pistil are deep in the tissues 

 and can not escape. When the pollen spore is carried to 

 the pistil it germinates by producing a tube which grows 

 into the ovary. The nucleus of the pollen cell divides once 

 or twice, and two or more of these nuclei pass down the 

 tube and reach the tissues in the ovary. In the meantime 

 the nucleus of the large spore in the ovary has divided 

 into eight or more nuclei, and one of these becomes the 

 nucleus of an egg. The nuclei in the pollen tube escape 

 and one of them unites with the egg nucleus. In this 

 way fertilization takes place. Thus we see that pollen 

 grains are not sperms, but nuclei in the pollen tube after 

 the pollen has germinated are really the sperms. The 

 egg is not formed directly by the growth of the ovary 

 tissues, but by the division of the nuclei in the large spore 

 which the ovary produced. 



4. What is the Real Difference Between the Old and New 

 Views? In the old view we have just a straight, direct 

 union of a male cell produced by a plant (such as an apple 

 tree) with a female cell produced by the same apple 

 blossom. In the new view we have the flower producing 

 spores as in the fern, only in the flower we have two kinds 

 of spores: a small spore produced in the stamen and 

 escaping, and a large spore produced in the ovule and not 

 escaping. The small spore (pollen) grows into a small 

 male plant (male gametophyte); the large spore in the 



