576 



BOTANY 



PAKT IT 



EMBRYO (Fig. 558), AND THE GROWING POINT OF THE SHOOT 

 ORIGINATES AT THE BASE OF THE DEPRESSION BETWEEN THEM. 

 MONOCOTYLEDONS, ON THE OTHER HAND, HAVE A SINGLE LARGE 

 TERMINAL COTYLEDON, THE GROWING POINT BEING SITUATED 

 LATERALLY (Fig. 559). In both cases the root is formed from the 

 end of the embryo which is directed towards the micropyle ; its 

 limits can be readily traced in older embryos. 



After fertilisation a considerable accumulation of reserve materials is necessary 

 in the embryo-sac both for the development of the embryo and for its future 

 use. It is thus of importance that a means of transfer of these materials 

 should exist. In the simplest cases the endosperm simply enlarges, crushing the 

 surrounding tissues of the nucellus ; often the antipodal cells, which are the 

 structures which lie nearest to the chalaza, are entrusted with the function of 



FIG. 558. Stages in the development of the embryo of Capsella bursa 

 pastoris (A-D). h, Hypophysis ; et, suspensor ; c, cotyledons ; p, 

 plumule. (After HANSTEIN, magnified.) 



FIG. 559. Young embryo 

 of Alisma Plantago. c, 

 Cotyledon ; v, growing 

 point. (After HANSTEIN, 

 magnified.) 



nourishing the embryo-sac. They then increase in number and sometimes undergo 

 considerable further development. Other portions of the embryo-sac may grow 

 out as long haustoria which sometimes emerge from the micropyle and some- 

 times penetrate the tissue beneath the chalaza. In some cases, especially in 

 insectivorous and semi-parasitic plants, a special store of reserve material is laid up 

 in this position for transference to the macrospore (Fig. 560). 



A further departure in the mode of development of the embryo-sac and embryo 

 is met with in some plants which live under quite peculiar conditions of life, such 

 as the Podostemaceae which occur in rushing mountain streams in tropical and 

 sub-tropical regions. In this case during the short dry period the immature 

 flowers rapidly develop at the cost of material which has been previously stored 

 up. Pollination, fertilisation, and the development of the seed are rapidly 

 effected in a shortened form, so that on the return of the aquatic conditions the 

 ripe seeds find the conditions for germination and serve to multiply and spread 

 the plants. 



In some cases plants have more or less completely lost the 

 capacity for sexual reproduction, which has been replaced by other 



