522 MANUAL OF BOTANY 



intermediate between those of their parents ; they are generally 

 fertile with either of the parent species, but not usually so with 

 another hybrid, or to a much less extent. When crossed with 

 one of the parent forms the offspring tend to revert to that 

 form. 



The result both of pollination and of fertilisation is generally 

 to stimulate the parts concerned to increased growth. In some 

 Orchids the ovules are not formed in the ovary until the stigma 

 is pollinated and seem to arise in consequence of that process. 

 The stimulus of fertilisation is still more marked. In the 

 Mosses its result is to cause not only development of the 

 sporophyte from the oosphere but a considerable additional 

 growth of the arohegonium, forming the calyptra. The 

 same thing may be noted in those Rhodophycese where a 

 bulky cystocarp is produced. The stimulus is, however, most 

 easily observed in the Angiosperms, where not only is the 

 oosphere excited to growth, forming the embryo, but the definitive 

 nucleiis of the embryo sac gives rise to the endosperm, and the 

 pistil or collection of macrosporophylls and sometimes other 

 parts of the flower undergo remarkable development, forming 

 the fruit. 



We have noticed that the asexual reproductive cell, whether 

 spore or gonidium, is generally found to remain in a state of 

 quiescence for some time after its formation. The same thing 

 is seen in the case of the zygote. In the Thallophytes this 

 resting period is sometimes a long one ; in the higher Crypto- 

 gams it is not so noticeable, and in the Phanerogams, where the 

 zygote is always developed inside the sporangium, it usually 

 proceeds to active growth at once. In the latter plants, however, 

 a resting period takes place later, after the seed is fully formed. 

 The development of the young sporophyte in fact takes place in 

 two stages, the one ending with what may be called the maturity 

 of the seed, the other beginning with the process of germination. 

 Seeds when detached from the parent plant preserve their 

 vitality for a variable length of time, sometimes even for years, 

 and are capable of germinating freely when exposed to favourable 

 conditions. 



The germination of the dicotyledonous seed occurs in one of 

 two methods. In the first of these the cotyledons are thick and 

 fleshy and remain under ground. As the seed absorbs water 

 and swells, the radicle makes its way out through the micropyle, 

 the testa bursts, and the plumule makes its way upwards, the 

 epicotyl, or part between the cotyledons and the first leaves. 



