REPRODUCTION OF PHANEROGAMS. 



623 



Thallophytes, as we have seen, great differences often exist in the 

 same plant even, in both asexual and sexual modes of reproduction, 

 the plant at one time and under one set of conditions reproducing 

 itself in one way, at another time and under different conditions 

 in another. For practical purposes such as the investigation of 

 parasitical plants detrimental to animals or to other plants, it is of 

 the greatest importance to know the life-history of the plants in 

 question and the conditions propitious or adverse to its several 

 modes of reproduction, as with such knowledge it may be possible 

 to devise a remedy or avert the mischief. 



Pollen-grains of Plianerogamia. 



Sperm-cells. Pollen-grains, the sperm-cells of Phanerogamia, 

 correspond to the microspores of the heterosporous Vascular Cryp- 

 togainia ; their protoplasm exhibits simply a nucleus, with granules 

 of starch, oily matters, and other ordinary cell-contents. In this 

 respect they approach the fertile cells of conjugating Alga?. 



Structure. In their simplest forms they are single cells, with 

 a proper cell-coat or intine, and an outer 

 cuticular coat or extine, mostly marked 

 with irregularities, forming a kind of 

 pattern on the surface, sometimes very 

 elegant. In particular cases the outer 

 coat is laminated, so that the pollen-cell 

 appears to have several coats. In all 

 cases the outer coat exhibits one, three, 

 or many round holes or slits (pores) 

 (fig. 602, ). where the inner coat is 

 bare ; in the pollen of Passiflora, Cucur- 

 bita, &c. there exist lid-like covers over 

 the pores (fig. 602, c). Bands or fur- 

 rows generally passing along the long 

 diameter of the pollen-cell are frequent, 

 varying in number according to the 

 species : but these bands are generally 



1 - Pollen-grams, magn. 1 00 diameters: 



a. Be 



gram 

 llis; 



b, Acacia laxa (com- 



only visible in the dry state. 



Form and Size. The form and size 

 of pollen-grains vary very much, as may 

 be observed even in the few examples 

 here figured; but although there may 

 be a general resemblance throughout particular genera, and even 

 Orders, they do not often afford good or regular systematic cha- 

 racters. They sometimes vary in different genera of the same 



pound grain) ; c, Passiflora ceeru- 

 lea ; d, Periploca graeca ; e, Trade 

 scantia ; f, Cichorium Intybus ; g, 

 Epilobium montanum ; h, Lathrcea 

 squamaria, forming pollen-tubes 

 among the cells of the stigma. 



