14 PART I. THE MORPHOLOGY OF PLANTS. [ 3. 



embryogeny is indirect or heteroblastic. In illustration of hetero- 

 blastic erabryogeny may be mentioned the Characese> where the 

 oospore gives rise to an embryo (sometimes termed proembryo) of 

 limited development, upon which the adult sexual form arises as a 

 lateral branch (Fig. 5) ; the Mosses, where the spore gives rise to 

 a filamentous embryo (the protonema), upon which the adult 

 Moss-plants arise as lateral branches : similarly, Lemanea and 

 Batrachospermum among the Red Seaweeds, where the spore gives 

 rise to a filamentous embryonic form (Chan transia- form) which 

 bears the adult form as lateral branches. Traces, more or less 

 distinct, of this mode of embryogeny are also to be found in the 

 development of the prothallium of some Ferns, which, in its early 

 stages at least, is filamentous and protonemal (see further under 

 Filicinae). 



Whilst the foregoing striking instances of heteroblastic embryo- 

 geny all refer to the gametophyte or sexual generation, indications 

 of a similar embryogeny are not wanting in the case of the sporo- 

 phyte. Among the lower plants something of the kind is offered 

 by Cutleria ( Phaeophyceae) where, from the zygospore, a club- 

 shaped body is developed, from which spring the flat horizontal 

 branches constituting the sporophyte. But a more important in- 

 stance is that of certain of the higher Pteridophyta (Selaginella 

 and Lycopodium) and of the Phanerogams generally, where the 

 oospore gives rise, on germination, in the first instance, to a more 

 or less elongated filamentous body termed the suspensor, at the ex- 

 tremity of which the embryo is eventually developed. 



From these forms the transition is easy to those in which the 

 embryo, whilst developing directly and continuously into the adult 

 form, possesses organs which are limited in their duration to the 

 embryonic stage : such are the foot of Muscineae and of most 

 Pteridophyta; the pegs or feeders of the seedlings of Welwitschia, 

 Gnetum, Cucumis, etc. ; and the primary leaves (cotyledons), 

 though in some forms they persist for a considerable time. 



Without entering at present into the details of embryogeny, it 

 may be pointed out that there are three principal modes in which 

 the development of the embryo from the spore may take place. 

 There is, first, that in which the spore grows out into a filament, 

 septate or unseptate, as generally in the Fungi, the filamentous 

 Algae, the gametophyte of the Mosses and Ferns, the male game- 

 tophyte (pollen-tube) of Phanerogams, the sporophyte of most 

 Phaneroga-ns (suspensor) ; secondly, that in which the spore grows 



