tapetum or megaspore-jacket[827] in the nucellus: immediately internal to the vascular bundles stretching from the chalaza to the apex of the megaspore-cavity is a layer of delicate cells with secretory sacs, and this is succeeded by a broad black layer of rather larger cells, a tissue which was probably in full activity in a younger stage of development. [Illustration: Fig. 493. A, _Lagenostoma ovoides_, longitudinal section of the lagenostome. B, _Lagenostoma Lomaxi_, transverse section of the lagenostome and canopy; _c_, pollen-chamber; _d_, space between the nucellus and integument. (A, after Miss Prankerd; B, from a section in the Manchester Museum, R 1048.) C, _Physostoma elegans_, longitudinal section showing microspores in the pollen-chamber, _c_. (From a section in the University College Collection.) D, _Physostoma elegans_, transverse section of the apical region showing the tentacles, pollen-chamber, _c_, etc. E, _Telangium Scotti_, transverse section of sporangia (Manchester Coll. R 1047). F, G, _Stephanospermum akenioides_. F, section of sclerotesta. G, tracheids from the nucellus. (University College Coll.)] A comparable tapetal layer is described by Lang[828] in the ovule of _Stangeria_: the majority of the sporogenous cells surrounding the megaspore become disintegrated and are absorbed, but the outermost zone forms a more definite tapetal layer: as already suggested, this tissue in _Physostoma_ may be a group-character. No archegonia have been found, but in a few cases some of the delicate prothallus-tissue occurs in the interior of the seed. Microspores are often abundant in the pollen-chamber (fig. 493, C, _c_); in one seed 80 are recorded. The occurrence of so many microspores suggested to Oliver that insect-agency may have been responsible for the precision in pollination that is greater than one would expect in anemophilous plants. The spores are smaller than those of _Lagenostoma_ (55μ × 45μ) and in several of them the remains of a cellular tissue are preserved (fig. 494, N), also some sub-reniform bodies (fig. 494, M) similar to those described as spermatozoids by Dr Benson in _Lagenostoma_ (fig. 408, D). The most interesting features of _Physostoma_ are: the absence of a continuous micropylar tube and its replacement by a circle of integumental lobes; the apical prolongation of the nucellar apex into the pollen-chamber, and the presence of long mucilaginous hairs on the integument. The large pollen-chamber is a character which distinguishes _Physostoma_ from _Conostoma_ and its form is very different from that in _Lagenostoma_. The tentacles of the integument and the form of the nucellar apex are features consistent with Oliver’s view that _Physostoma_ is the most primitive of Palaeozoic seeds though, as Burlingame[829] says, the elaborate form of the encasing envelope marks a considerable advance beyond the earliest type of megasporangium integument.