ZOOLOGY AND BOTANY, MICROSCOPY. ETC. I r ^'.> 



Avicennia are especially remarkable ; they spring from the horizontal 

 suckers, and rise erect into the air to the height of half a metre, with a 

 •diameter of O'5-l cm. ; they often branch, ami bear a large number of 

 pneumathodcs or lenticels. 



j8. Physiology. 

 (1) Reproduction and Embryology. 



Relationship between Parasitism and Sexual Reproduction. — 

 Prof. M. Moebius* calls attention to the fact that in both cryptogams and 

 phanerogams the parasitic or saprophytic habit is very commonly 

 associated with a limitation of the functions or even with a complete 

 suppression of the organs of sexual reproduction. This is the case 

 throughout the whole class of Fungi, and with many of the parasitic 

 flowering plants, such as the Balanophoracea3,Santalacea?,and Rafflesiacea?. 

 He suggests an antagonism between the two physiological processes or 

 conditions. 



Commenting on this suggestion, Prof. K. Gcebel "j" points out that 

 Balanoplwra is at present the only known instance of a complete loss of 

 sexual reproduction in flowering plants. Examples of apogamic embryo- 

 formation are not uncommon ; but these are by no means confined to 

 parasites and saprophytes (e.g. Allium, Coelebogyne, Funkia). 



Double Impregnation in Angiosperms + — A further series of obser- 

 vations leads M. L. Guignard to the conclusion that the j>rocess described 

 by him as double impregnation is a usual phenomenon in Angiosperms. 

 He has observed it in members of the orders Ranunculaceaj, Resedacete, 

 Malvaceae, and Composite. In all the examples the secondary jiucleus 

 of the embryo-sac is formed, by comjdete fusion of the polar nuclei, a 

 considerable time before impregnation. The fusion of the antherozoids 

 with this nucleus and with that of the oosphere takes place with great 

 rapidity. 



In Caltlia and Ranunculus one of the male nuclei applies itself, in 

 the form of a crescent swollen in the centre, to the nucleus of the 

 oosphere, from which it is easily distinguished by the greater capacity 

 for staining of its chromatic granulations. In Anemone nemorosa the 

 very large secondary nucleus is usually found towards the base, and near 

 the well-developed antipodals. The antherozoids are small elongated 

 slightly curved bodies. In all the Ranunculacea! examined, a largo 

 number of endosperm nuclei are formed before the first septation of the 

 embryo. In Reseda lutea the nucleus of the oosphere, comparatively 

 poor in chromatin, is smaller than those of the synergids ; its single 

 nucleole is also smaller. In Rudbeclia the antherozoids are not 

 elongated and curved, as they are in Hclianihus. 



Embryo-sac of Monocotyledons.§ — Karl M. Wiegand makes the 

 following obervations on the development of the embryo-sac in certain 

 monocotyledonous plants. 



Convallaria majalis. The stages of the growth and development of 

 the archesporial nucleus are identical with those of the nuclei of the 



* * Biol. Centralbl., xx. (1000) pp. 500-71 (5 figs.). t Tom. cit.. pp. 571- 2. 

 , % Comptes Eendus, exxxi. ( J 900) pp. loo-fiO. Cf. this Journal, ante, p. 481. 

 § Bot. Gazette,, xxx. (lyOO) pp. 25-47 (2 pis.). 



