388 



FOURTH GROUP. — SEED-PLANTS. 



Gymnosperms is the archesporium, but it undergoes only a few divisions. Of the 

 sporogenous tissue which is thus formed, and which in Polygonum is composed of four 

 cells, one cell displaces the others and becomes the macrospore or embryo-sac. The 

 antipodal cells are a rudimentary prothallium, but it at present remains uncertain 

 whether the egg-apparatus can be looked upon as a rudimentary formation of 

 archegonia. 



The structural characters of an anatropous ovule have already been briefly 

 described and illustrated; Fig. 321 shows its development. The ovule in this case 

 consists of only a single axile row of cells surrounded by an outer layer of cells. 

 The inner integument is seen in process of formation in Fig. 321 /, //. The mother- 

 cell of the embryo-sac here, as in many other cases, gives off no primary tapetal cell 



above. It divides into three 

 daughter- cells, the lower of 

 which displaces the upper ones 

 and developes into the embryo- 

 sac, in which the egg-apparatus 

 and the antipodal cells are 

 formed exactly as in the in- 

 stance of Polygonum which we 

 have just examined ; but the 

 antipodal cells disappear at an 

 early period (Fig. 321 VI, g). 

 In other cases the growth and 

 repeated division of the primary 

 tapetal cell, which lies above 

 the mother-cell of the embryo- 

 sac and of its daughter- cells, 

 makes the embryo-sac descend 

 deep into the tissue of the 

 nucellus. This proceeding which puts us in mind of the Coniferae, but does 

 not seem to be very common in Angiosperms, may be illustrated by Mercurialis 

 annua (Fig. 320). The occurrence moreover of two or more mother-cells of 

 embryo-sacs, produced perhaps by the division of a single one, is not 

 very rare ; they are found for instance in Chrysa7ithe?nu?n Leucanthemum, 

 Hellehorus cupreus, Thesium intermedium and some species of Rosa^ etc. In Rosa 

 several embryo-sacs have been found even in the mature ovule. Strasburger saw four 

 hypodermal embryo-sac-mother-cells in the young ovule of Rosa livida, and each of 

 them gave off a primary tapetal cell above. These primary tapetal cells, like the cells 

 of the nucellus above them, divide further by transverse walls, so that a cap of tissue 

 lies over the embryo-sac-mother-cells (archesporial cells), each of which then divides 

 by repeated bipartitions into from one to six daughter-cells, and the uppermost 

 usually of these, not as is the rule in other cases the lowest, becomes the embryo-sac ; 

 sometimes both the upper cells become embryo-sacs, which then destroy the 

 circumjacent tissue. 



This case shows that each of the cells, into which the archesporium or embryo- 

 sac-mother-cell is divided, may under certain circumstances develope into an embryo- 



Loiigitudinal sections through thenucelli 



of the 



Fig. 320. Mercurialis 

 ovule, to the right a young, to the left an older stage of development. A the mother- 

 cell of the embryo-sac (archesporium) ; the primary tapetal cell 5 above it has divided 

 into three cells, which still show considerable growth and divide by transverse and 

 longitudinal walls. By this means the mother-cell of the embr>-o-sac is sunk deep in 

 the tissue of the nucellus. In the left-hand figure it has divided by transverse 

 walls into three cells, the lowest of which supplants the other two and becomes the 

 embiyo-sac, J integument. After Jonsson. 



