REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



246 



increases by the repeated division of a ter- 

 minal cell by alternately inclined septa. The 



Fig. 182. 



77te same after impregnation, 150 diam. 



The papilla has been broken off in making the 



preparation. 



direction of growth of this structure, which is 

 the first leaf of the embryo, is obliquely up- 

 wards. In its axil is formed the primary axis } 



Fig. 183. 



of the vertical series of sporangia which cor- 

 responds to each valve, one only of the ori- 

 ginal central cells continues its development, 

 the rest becoming abortive, and finally disap- 

 pearing. The four spores, which are formed 

 just as in Selaginella, at length become free 

 by the absorption of the cell in which they 

 are enclosed, and for a time continue to en- 

 large equally, while their walls are thickened 

 by internal gelatinous deposition. Soon, how- 

 ever, one begins to exceed the rest in growth, 

 and finally occupies the whole cavity of the 

 sporangium, which is subsequently burst by 

 the swelling of the exospore, which is pro- 

 duced when it is subjected to the influence of 

 moisture. 



101. The microspores are developed precisely 

 as in Selaginella. The exosporal membrane 



Fig. 184. 



Embryo of Pilularia alobulifera, 10 diam. The 

 embryo is still enclosed in the prothallium, the tissue 

 of which has expanded so as to form an investment 

 for it. 



a, remains of papilla of archegonium with its canal ; 

 b, first root; c, first leaf; d, primary axis; 

 e, cavity of naacrospore. 



and soon after, as a lateral development from 

 this last, the second leaf. In the meantime 

 the first root makes its appearance as a rounded 

 projection, which grows from the upper sur- 

 face of the embryo, in a direction opposite to 

 that of the first leaf. Both of the last-men- 

 tioned organs finally burst through the remains 

 of the prothallium, and become free. 



100. Sporangia and spores. The organ in 

 which the sporangia of Pilularia are contained 

 is an egg-shaped body, supported on a short, 

 curved pedicle, which springs directly from 

 the creeping stem, in the axil of one of the 

 awl-shaped leaves. It presents a tough, cori- 

 aceous, cellular coat, which encloses a cavity, 

 which is divided into four compartments by 

 vertical septa, and subsequently dehisces in 

 four valves. The middle of the internal sur- 

 face of each valve is, from the first, marked by 

 a ridge of gelatinous cellular tissue, from which 

 the sporangia take their origin as a vertical 

 series of projections. Their development re- 

 mains up to a certain time the same, whether 

 they are to produce large or small spores. All 

 are found to exhibit at this period a central 

 mass of cells, containing nuclei and grumous 

 fluid, which is surrounded by a double capsular 

 layer. In each of the central cells, the nucleus 

 soon after is replaced by four others of 

 smaller size, around which are formed four 

 tetrahedral secondary cells, which are the im- 

 mediate parents of the spores. In the lowest 



Microspore of the same, 600 diam. 

 The inner membrane projects through the outer, 

 which has given way. A few of the cellules 

 containing spermatozoids have escaped. 



dehisces in three valves, the proper membrane 

 of the spore at the same time giving way irre- 

 gularly, to allow the escape of numerous little 

 globular cellules. These cellules contain, in 

 addition to starchy and mucous granules, pa- 

 rietal lenticular vesicles, each of which en- 

 closes a delicate, spirally coiled antherozoid, 

 which moves actively in water. 



102. Phanerogamia. Between the higher 

 vascular Cryptogamia, and the simplest forms 

 of flowering plants, there exists, as has been 

 already noticed, a wide chasm of obscurity. 

 The researches, however, of Hofmeister, have 

 shown that in the Coniferas the embryo is 

 formed upon a plan which presents the most 

 striking analogies to what is observed among 

 the Rhizocarpeas and Lycopodiaceas ; and that, 

 in fact, their development stands intermediate 

 between that of the plants just mentioned 

 and the angiospermous Phanerogamia. 



103. Phanerogamia gymnospermia. Follow- 

 ing the same plan of description that we have 

 adopted in the previous section, we shall confine 

 our attention to the Abietinese, of the deve- 

 lopment of which Hofmeister has furnished us 

 with a most complete account. The so-called 

 ovule consists, at the time of the scattering of 

 the pollen, of a short and thick nucleus of 

 delicate cellular tissue, which is enclosed in a 

 single, somewhat fleshy integument, leaving 

 onen a wide micropvle canal.* In the centre 



open a 



* For the origin and signification of botanical 

 terms in common use we refer the reader to any of 

 the elementary works on Botany. 



