FERTILIZATION AND FRUIT-FORMATION IN CRYPTOGAMS. 55 



It is no more possible to say which of the two protoplasts uniting in the zygo- 

 gonium of Sporodinia grandis is fertilized and which acts as fertilizer, than it is 

 to predicate of the pairing protoplasts of Ulothrix, that the one is the ooplast, and 

 the other the spermatoplast. Theoretically we must assume there is a difference, 

 and it probably consists in peculiarities of molecular constitution, but no perceptible 

 difference can be detected in size, configuration, or colour, nor is there any apparent 

 distinction in respect of origin. 



In the DesmidiacecB also, of which two examples {Closterium and Fenium) are 

 given in vol. i. Plate I. figs, i and k, and in the DiatoTnaoeoe, whose species are 

 reckoned by hundreds, no perceptible external difference exists between the proto- 

 plasts which unite for the purpose of fertilization. Only in the ZygnoemaceoB is it 

 possible to look upon a particular one of the combining protoplasts as an ooplast, 

 and the other as a spermatoplast, and the distinction is in this case founded on 

 their relative positions. An instance of the mode of fertilization prevailing in these 

 plants is shown in Plate I. fig. I, in the first volume, the case chosen for illustration 

 being that of Spirogyra arcta, which consists of green filaments of a slimy con- 

 sistence, and occurs very commonly in our ponds. The cells are arranged in 

 linear series, and from some of them are formed lateral outgrowths like those 

 produced by the tubular cells of Sporodinia grandis. As in Sporodinia, the out- 

 growths from opposite cells come into contact, coalesce, and form a kind of yoke. 

 Usually a number of the opposite cells of two filaments floating close together in 

 the water establish connecting links of the kind, which resemble the rungs of a 

 ladder (see vol. i. Plate I. fig. I, to the right). The wall formed by the coalescence 

 of the two apices of the outgrowths is removed by solution, and a channel con- 

 necting the opposite cell-chambers of the Spirogyra-&la,ments is thus opened up. 

 In the meantime the protoplasm in each of these cells undergoes a change. 

 Hitherto it has been occupied by a chlorophyll-body in the form of a spiral band, 

 but now it assumes the form of a dark-green spheroidal mass, which is destined to 

 unite with the one opposite to it. In Spirogyra this coalescence does not take 

 place in the middle of the connecting canal as in Mucor and Sporodinia, but the 

 green ball of protoplasm from one cell glides through the transverse passage into 

 the opposite cell-chamber, and there coalesces with the second protoplasmic mass 

 which has remained at rest and hot changed its position. It is permissible to call 

 the resting protoplast an ooplast, and the one which moves across to it a spermato- 

 plast; but it must again be expressly stated that in Spirogyra no difference in size, 

 shape, or colour can be detected between the two uniting protoplasts. It is worthy 

 of note that the zygote produced by the coalescence, and now assuming an ellip- 

 soidal shape, is not equivalent in bulk to the two protoplasts, as one might expect, 

 but that its volume is obviously smaller. We may infer from this that at the 

 moment of coalescence a fundamental change in the molecular structure of the 

 entire mass takes place. The characteristic property of fertilization in the Gon- 



juqatcB of which Sporodinia grandis and Spirogyra arcta have here been chosen 



as examples consists in the union of two separate individuals by means of the 



