ioo PRIMITIVE FORMS OF LIFE 



Horse-tails their name of vascular Cryptogams. When a cluster 

 of leaves develops at the same point on the rhizome, these 

 leaves become adherent and form a secondary stem, which 

 grows erect, as with the tropical tree-ferns, and probably also 

 at first with the Horse-tails with verticillate leaves. In the case 

 of the Mosses, it is the leafy stem which bears the organs of 

 sexual reproduction, and also a kind of accessory plant fixed 

 on this leafy stem which furnishes the sporangia and the spores. 

 In the vascular Cryptogams we find a singular reversal of the 

 dimensions of the sexual and the asexual plants that alternate 

 regularly in the development of Mosses, and up till the present 

 no intermediate condition has been found to fill the gap 

 separating the latter from the Cryptogams. The sporangia 

 are, indeed, borne on the large leaves of the Ferns and on the 

 leaves of the accessory stems of the Club-moss and the Horse- 

 tails ; the spores arising from these sporangia give birth only 

 to a leaf-like shoot without roots, resembling the thallus of the 

 Hepaticse and known as the prothallus. This pro thallus bears 

 archegonia, each containing an oosphere, and antheridia which 

 produce antherozoids. Every fecundated oosphere gives rise 

 to a new leaf -bearing stem. 



The same process takes place in the three classes of Ferns, 

 Club-mosses, and Horse-tails : in all three there are parallel 

 modifications of reproduction, due to tachygenesis and to the 

 gradual transformation of the normal method of reproduction 

 into another more accelerated method, characteristic of the 

 gymnospermous Phanerogams. From this we can draw the 

 inference that it is certainly tachygenesis which transformed the 

 vascular Cryptogams into gymnospermous Phanerogams, though 

 probably each class of Cryptogams has passed over separately to 

 the gymnospermous condition, and given rise to special types of 

 the latter. Indeed, the Cycads, with their large leaves, seem to 

 be connected with the Ferns ; the Conifers, with their small 

 leaves arranged spirally, with the Club-mosses ; and the 

 Gnetaceae, with their small whorled leaves, with the Horse-tails. 



However that may be, the course of tachygenesis in the three 

 classes of Cryptogams is the same. 



i. The dimensions of the prothallus are reduced and instead 

 of developing outside the spore, they develop inside it. 



2. The sporangia, instead of being identical in form, are 

 divided into two groups : the macrosporangia, which produce 



